What does second cousin first removed mean?

The two terms describe two different relationships.

Your first cousins are the children of your aunts and uncles.

Your second cousins are the grandchildren of your grandparents' brothers and sisters.

You have two kinds of first cousins, once removed.

1. The child of the brother or sister of one of your grandparents is your first cousin, once removed. This person is first cousin to one of your parents.
2. The child of your first cousin is your first cousin, once removed. This is the grandchild of the brother or sister of one of your parents.