What does ben mean before a surname?

"Ben" means "son of" in Semitic languages like Hebrew and Arabic. It is used before a patronymic, not before a surname.

A surname is a family name that is handed down from generation to generation.

A patronymic is the name of the father, and changes with each generation.

For example, if David the son of James has a son named Sam, and Sam has a son named Frank, then using patronymics, David is David ben James, Sam is Sam ben David (or Sam Davidson) and Frank is Frank ben Sam (or Frank Samson).

At times, patronymics get converted into surnames. Then, with the example above, if Sam lived when such a conversion happened the whole "ben David" becomes the surname and Frank is Frank benDavid, despite the fact that his father is Sam.