Romans 4:2

If, in fact, Abraham was justified by works, he had something to boast about—but not before God.

(v2, NIV)

Abraham would have had something to boast about if he was justified by works. Justification by works brings forth the possibility of boasting. However, this is not the case with justification by faith. Justification by works is what most people think is the way to be saved. People think if you do a bunch of works, you can merit something allowing you to be justified. What's interesting is that this hypothetical situation Paul presents is "not before God". If justification came by works, you could boast, but not before God. The boasting allowed to one who is justified by works doesn't even apply before God. Before God, even this hypothetical boasting cannot stand.