Definition of brag
A brag is an English term meaning "to boast". In other words, a brag is an action that one does or a story that one tells with the simple and unique goal of impressing people. We can then talk about "brag" to make it a verb. "Yesterday, I finished 1st in a tournament and I won 100,000 euros". A "brag" is something you are proud of and want to show off to as many people as possible. Driving around in a 200,000 Euro convertible with music blaring in order to get noticed is what you might call a brag.
Brag and poker
Some poker players are very proud. Have you ever watched the World Series of Poker on TV (the WSOP)? When you have a very proud and confident young player who is on the same table as a poker legend like Phil Ivey or Daniel Negreanu, he may try to bluff them. His goal is to show his bluff and say "I got you, it was a bluff!" and then tell his friends the story: "You don't know what? I bluffed Phil Ivey at the WSOP! That's what we call a poker brag!