The phrase "triangle button" refers to a button on a video game controller that is shaped like a triangle. It is used to perform specific actions or functions in the game. Full definition
Check and make sure that the face button's have got uniform height, and the circle and triangle buttons in particular are correctly lined up. (playstationing.com)
The controls for the game are pretty straight - forward, with the circle and triangle buttons issuing most of the attacks you'll deliver in the game. (thevitalounge.net)
You can even do special moves by just pressing the square and triangle buttons together. (justpushstart.com)