Asere is Cuban slang which literally means "I salute you" and is commonly used to mean friend, buddy or dude.
It's origins are not Spanish but rather Efik—the language spoken by the people of the same name in southeastern Nigeria. (The Efik language is a variation on the Ibibio language.)
Today it is part of a common greeting in Cuba: ¿Que bola, asere?
The word asere is also used in Puerto Rico in much the same way.