It's an Italian adjective which is used to compliment or congratulate someone for something he (For the "she" part, see 2. below) has done or achieved or just said.
There are some minor differences in how it's used by native Italian speakers compared to how it's used in the US and other English speaking countries, though.
1. the word "bravo" in the US is mainly used in the context of artistic performances and public exhibitions (or, with a touch of irony, in informal speech), while in Italian it can be used more generally to denote someone's ability in some area or specific well-doing (see examples below).
2. in the US "bravo" is essentially used as it were an interjection and it's rarely conjugated, while in Italian it is an adjective and as such it must be conjugated:
bravo ---> male, singular
brava ---> female, singular
bravi ---> male, plural
brave ---> female, plural.
Strictly speaking, it's a mistake to say "Bravooo!!" while applauding the performance of a female violinist or of a male rock band: you should say "brava!" in the first case and "bravi!" in the second.
Mario è bravo in matematica ( = Mario is good in maths)
Angela è brava a suonare il piano ( = Angela is good at playing the piano)
Braviii!!! (applauding the Kronos String Quartet)
Braveeee!!! (applauding the female cheerleaders)