It's the Italian female equivalent of the male adjective "Bravo".
it is then referred to a woman to compliment and approve her action.
At the theater you should yell "BRAVA!" to congratulate a Soprano or other woman singer, not bravo (that's for a man)!
Bravissima is the superlative for a woman, so you can yell that to. (Bravissimo for a man)
e.g. Brava! Ottimo lavoro!
(Brava! excellent job!)
e.g. Ms. Maria Callas was such an excellent soprano that people acclaimed yelling her, "Bravissima!"