It has two principle meanings:
1) Something is ready or finished, short for "ya esta listo." : La comida ya esta - dinner is ready. Ya estas? Are you ready (to leave, dressed, etc.). Cuändo va a estar el informe? When is the report going to be done? Todo ya estä, solo faltan los globos. Everything is ready - all we need is balloons.
2) "And that's it, and that's that, and that's all there is to it." When you talk about a final solution: Pagamos lo que haya que pagar y ya estä. We'll (just) pay what we have to pay and that's that. Lo pintas todo de blanco y ya estä. Just paint it all white and that'll be the end of that. Hoy vas a la escuela y ya estä. You're going to school today and that's final. etc..