"Holla" is a word used to acknowledge someone's presence, like when somebody just shows up: "Holla! My girl is finally in the house!" Although this is its primary meaning, "holla" is also used a few other ways.
The word "holla" can also be used to express interest in someone, as in, "Holla! That's one fine girl coming up in here." "Holla" also has the same meaning as the word "holler," a common expression used in the South that means to contact somebody. One example of this usage would be, "Holla at me later to see if we can hook up."