To shout/give a shout out/verbally call in order to make your presence notable by someone or show that you offer your help to someone in case they ever need it. In some occasions it might reffer to a cry for help. Included in one of Tupac Shakurs' songs "Holler If Ya Hear Me"
"Holler if u need anything"
"Yo holler at me, ok?"
"He tried to holler at me"