Cousins share a pair of grandparents in common; second cousins share a pair of great-grandparents; third cousins share a pair of great-great-grandparents. “Once removed” means that the two cousins are not in the same generation, but are one generation apart — e.g., the common ancestors are one cousins grandparents but the other cousins great-grandparents. “Twice removed” means two generations of difference — e.g., the common ancestors are one cousins grandparents but the other cousins great-great-grandparents. And so on.

Idiomatically, it means that he is very distantly related. It means that if you looked at a complete family tree, he would be on there somewhere. But it also implies that you have never met them or spent time with them or know them or recognize them or their immediate family. Technically, the number-cousin shows how many generations you are from a common ancestor and "removed" means that there's a difference in generation between the two people. So, this sentence *technically* means that either his grandparent is your great-great-great grandparent or that his great-great grandparent is your great-great-great-great-great-great grandparent. But most people don't know the technical meaning. They would use the idiomatic meaning that "this guy and I are related...somehow"