What is a second cousin?

Someone with whom your most recent common ancestor is a great grandparent to both of you.

Like how a first cousin is someone with whom your most recent common ancestor is a grandparent to both of you (aka you both have a same grandparent but not a same parent - meaning one of each of your parents are siblings with each other).

If the most recent common ancestor isn’t the same relation for both of you, then there’s probably a “times removed” or one of you is some variety of aunt/uncle to the other.