15 Brethren, I speak in terms of human relations: even though it is only a man’s covenant, yet when it has been ratified, no one sets it aside or adds conditions to it. - Galatians 3:15

“even though it is only a man's covenant” Paul is pointing out that even a man's contract (covenant), once it's been signed (ratified) is not then changed thereafter. “I speak in terms of human relations” Understanding that even with a man's covenant nobody adds conditions to it after it's been ratified. The contrast here is that this covenant was God's covenant and not man's and therefore this agreement is backed by the highest possible power, God Himself. “no one sets it aside or adds conditions to it” An agreement is an agreement, you cannot discard it nor modify it after it's been ratified.

16 Now the promises were spoken to Abraham and to his seed. He does not say, “And to seeds,” as referring to many, but rather to one, “And to your seed,” that is, Christ. - Galatians 3:16

“the promises were spoken to Abraham” Such promises as those written in Gen. 22:17-18.

17 indeed I will greatly bless you, and I will greatly multiply your seed as the stars of the heavens and as the sand which is on the seashore; and your seed shall possess the gate of their enemies. 18 In your seed all the nations of the earth shall be blessed, because you have obeyed My voice.” - Genesis 22:17-18

“and to his seed...that is, Christ” The covenant made with Abraham was to Christ. Christ was the utmost beneficiary of God's promise to Abraham. As we read earlier in Galatians, Abraham trusted in this promise and God credited it to him as righteousness. Abraham was able to see the promise of God by faith even though it had not yet come to pass.

56 Your father Abraham rejoiced to see My day, and he saw it and was glad.” - John 8:56

The Hebrew word for seed in Genesis is zera which is a collective singular and is used 229 times in the old testament. Christ is the seed, and all those in Christ are the collective. It is actually because of the covenant with Christ that even Abraham was made a beneficiary. Not the other way around. Christ was the firstborn among many brethren (Rom. 8:29). However many Jews thought they would be blessed because they were descendants of Abraham, they felt the promise to Abraham applied to all in Abraham's lineage which would then mean seeds (plural) instead of seed (singular).

Paul is pointing out that this promise was given to Abraham and his seed (singular) and not seeds (plural). This was an earth shaking revelation for many of the Jews who trusted in “their father Abraham” (see also Matt. 3:9, Luke 3:8, 16:24, John 8:33, 53)

39 They answered and said to Him, “Abraham is our father.” Jesus *said to them, “If you are Abraham’s children, do the deeds of Abraham. - John 8:39

A plea to not only see the futility in claiming Abraham's lineage but to trust in God as Abraham trusted in God.

In Genesis we read that God establishes this promise through the lineage of Isaac and the idea that the promise applies to all of Abraham's decendants is debunked.

18 And Abraham said to God, “Oh that Ishmael might live before You!” 19 But God said, “No, but Sarah your wife will bear you a son, and you shall call his name Isaac; and I will establish My covenant with him for an everlasting covenant for his descendants after him. 20 As for Ishmael, I have heard you; behold, I will bless him, and will make him fruitful and will multiply him exceedingly. He shall become the father of twelve princes, and I will make him a great nation. 21 But My covenant I will establish with Isaac, whom Sarah will bear to you at this season next year.” - Genesis 17:18-21

The genealogy of Jesus is brought out early in the gospel accounts of Matthew (1:1-17) and Luke (3:23-38) and for good reason. It records the fulfillment of the promise God gave to Abraham through the lineage of Isaac, that is Jesus Christ. To gentiles this may not seem significant but to the Jewish people this was huge.

17 For truly I say to you that many prophets and righteous men desired to see what you see, and did not see it, and to hear what you hear, and did not hear it. - Matthew 13:17

Paul so easily unveiled (through the revelation of Jesus Christ) what was once, for many, a great mystery.

17 What I am saying is this: the Law, which came four hundred and thirty years later, does not invalidate a covenant previously ratified by God, so as to nullify the promise. - Galatians 3:17

Paul is pointing out that the law cannot and does not invalidate a covenant previously ratified by God. This was very important for his Galatian brethren (v. 15) to understand since they were adding law based conditions to the promise given to Abraham. A promise that was given 430 years before the law even came into existence and therefore had no part in the original contract between God and Abraham. If the law which came 430 years later was allowed to be added to the contract after it had already been ratified, just like a human contract today, the entire contract would be made void since conditions were added after both parties signed it.

For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise. - Galatians 3:18

God's blessing is no longer a blessing if it's earned, it becomes wages as what is due (Rom. 4:4). If the inheritance is based on the law (as if one could some how keep the entire law as to obtain it even if it was) then it's no longer God's blessing ('God's blessing' means the good things that God gives to His people).

23 For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord. - Rom. 6:23

The wages of the law (which exposes sin) is death. The promise of Abraham is given freely by God to Jesus Christ and all who are in Him.

The word granted in Gal. 3:18 is the Greek word kecharistai, which is based on the Greek word charis which means grace or favor given. God's giving to Abraham was the free giving of grace. The word is also in the past tense, showing that the gift is done, permanent, ratified.