Sr. is short for "Senior". This means that Tim Russert has a child with the same name. So he is Tim Russert Sr. and his son is Tim Russert Jr. (Junior). In English if a parent and their child share the same name, the parent gets a Sr. after their name and the child gets Jr. This is so others can distinguish the two people. I hope this is clear!

Senior. It means he also has a son with the same name.

Senior. He presumably has a son called Tim Russert Jr. (junior).