It’s an exclamation. An archaic use to call attention to something.

'Holla' is a colloquial way to spell/say the old-ish word 'holler.' It literally means 'to yell' but lot's of people use 'holla' to mean 'talk to me' or just as meaningless expression. Holla!

Or try to use Google translate.

He doesn't say holla, he says "I'll, uh, text you...".