"Brothers, let me take an example from everyday life. Just as no one can set aside or add to a human covenant that has been duly established, so it is in this case." NIV translation
A covenant is a contract where the details of a relationship are set out. Most contracts are witnessed by two people and, once it is signed (duly established), it is binding. Today, it is usually done in writing but, for most of history, a man's spoken word was as good as a signature. With this in mind, Paul is going to apply it to the Abrahamic covenant.
"The promises were spoken to Abraham and to his seed. The Scripture does not say 'and to seeds,' meaning many people but 'and to your seed,' meaning one person, who is Christ." NIV translation
God's promises to Abraham were a contract that could only be fulfilled by Jesus Christ. Although it is tough for many to accept, there is only one way and that is Jesus Christ.
Many want to claim Abraham as the father of their faith but, even though he had another son (Ishmael), the promise was not spoken to him.
"What I mean is this: The law, introduced 430 years later, does not set aside the covenant previously established by God and thus do away with the promise." NIV translation
The law did not change the promise but was simply an instrument that God used in fulfilling the promise. The purpose of the law was to make us understand that we need Jesus. Jesus was the promise that God made to Abraham and, through Jesus, we all have the ability to be children of that promise.
"For if the inheritance depends on the law, then it no longer depends on a promise; but God in his grace gave it to Abraham through a promise." NIV translation
When you try to add any works to grace, then you no longer have grace. Neither Abraham nor us did anything to deserve the promise and we cannot lose the promise because of something that we do. This is the same struggle that many face even today as we, as people, want to earn salvation but cannot.