Definition of brag
A brag is an English term meaning "to brag". In other words, a brag is an action that is done or a story that is told with the simple purpose of impressing people. We can then talk about "to brag" as a verb. "Yesterday I finished first in a tournament and I won 100,000 euros". A "brag" is something to be proud of and to be spread out to as many people as possible. Driving around in a convertible at 200,000 euros with music at the top of your lungs in order to get yourself noticed and what you could call a brag.
Brag and poker
Some players of poker are very proud. Have you ever watched the World Series of poker on TV (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 might try to bluff them. His goal is obviously to show his bluff and say "I got you, that 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 brag out of poker!