This is a really easy to learn expression that means basically, "done" or, "it's a done deal". You will get big bonus points using this. But save it for some situation where you really want to be cool like,
Let's have lunch together sometime? Já é!
Já é literally means, It already is.
Vinte reais está bom? Já é!
Vamos tomar uma cervejinha? Já é!