A more thorough explanation:

FIRST COUSIN

A first cousin is a relative who shares a grandparent with you. They are the children of your parents' siblings.

For example, if your father has a sister and she has a son, that son is your first cousin. Similarly, if your mother has a brother and he has a daughter, that daughter is also your first cousin.

Another example would be if your aunt on your mother's side has a son, he would be your first cousin.

These examples illustrate how first cousins are related through their grandparents and their parents' siblings.