This term means to yell at some hot bitches down the street of your hood. Some do this with a gangsta love attitude, if you will. Most people do this to a so called "Butterhead" because "Butterheads" are known to have a nice ass.
"yo... look at dat butterhead down the street, lets halla at dat bitch"
"hey bitch!... wanna suck my ballz?"
"yo... you have a great ass... halla"