Definition of brag
A brag is an English term meaning "to boast". In other words, a brag is an action one does or a story one tells simply to impress people. We can then talk about "bragging" to make it a verb. "Yesterday I finished 1st in a tournament and won €100,000. 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 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 obviously to show his bluff afterwards and say "I got you, it was a bluff!" and then go 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!