A service fork is a fork you use for silver service. Basically, it's a fork which you use to serve food (along with a spoon). Are you learning silver service now?

an instrument having two or more prongs or tines, for holding, lifting, etc., as an implement for handling food or any of various agricultural tools. Meaning: Cutlery used for serving and eating food.

service fork is such as a menu buffet in a restaurant

Actually service fork is a fork you use for silver service. you can use this to serve food .