What does estas aki mean in English?

Answer 1

estás aquí means you are here

Éstas aquí --- these ones here (noun in fem. gender).

The importance of accents in Spanish is to show the difference in pronounciation, and therefore in grammar category, between words:

Ésta (this one, fem gender)

Está (You are, polite singular)

Answer 2

Aqui estoy yo means 'here I am' in English. It is a common phrase and also a name of a song. The yo is not necessary in the phrase because estoy already means 'I am'.