What is Jr or Sr?

Jr is Junior and Sr is Senior.

They are often added to names when a father(Sr) and son(Jr) share the same name. They are also used when two people with the same name are not father and son. For example a grandfather and grandson or an uncle and nephew might have the same name and be in the same communinty.

The purpose is to say which is the older and which the younger, and so to tell the difference between two people with the same name.