From the urban dictionary:
holla:
1. A word used to acknowledge the presence of a fellow companion
2. For a man to express interest in a particularly impressive female specimen
3. To contact via telephone
ex1. Is that mah boy ova there? HOLLAAAAA!
2. Watch out, I'm bout to holla at this fine b1tch.
3. Boy, it was absolutely magnificent to see you again. Holla at me this evening, we can have tea and crumpets.
I'm guessing it's door number 1.
If so, holla back at'cha.