Brawn is a character's physical strength, quickness, dexterity, stamina, constitution and health level. There are two Brawn scores: the Brawn Maximum Capacity which designates the highest a character's Brawn can naturally go up to when they are fully healthy, and the Brawn Pool which shows how many Brawn points are available to the character at any specific time. A character's Brawn Pool fluctuates according to how much physical damage they have recently sustained. The Brawn Pool is the base score for use in combat bidding.
During play a character will use up the points in their Brawn Pool. When you spend points from your character's Brawn Pool you must subtract those points from the Brawn Pool score on the attribute card (pictured above). Your character will also have the opportunity to restore points to their Brawn Pool. The circumstances in which you get to do this are described in the Monster Manual description for your character's species.