How is HBr an acid?

Answer 1

There are several definitions of acids. The most useful in school-level chemistry is the Lowry-Bronsted definition which says that an acid is a proton donor. The HBr molecule (which you would regard as covalently bonded) breaks up or dissociates into an H+ ion and a Br- ion. So the proton (the H+) can then attach itself to other things, ie it has been donated to something else, for example if it attached to a hydroxide ion OH- it would form water. This is what happens when an acid neutralises an alkali.