Kinship terms in English begin with the first common direct ancestors. People who have grandparents in common are first cousins; great grandparents, second cousins, and so on.
It gets a bit tricky when people are of different generations. If Person A is the great grandparent of Person B but the grandparent of Person C, then B and C are first cousins once removed, i.e. the difference between a great grandparent and a grandparent is one generation. So your parents’ first cousins are your first cousins once removed. In other languages, such as German, these are called aunts and uncles as well.
To look at it another way, your grandparents’ siblings are your great uncles or great aunts — more rarely, grand uncle/aunt. Their children are your parent’s first cousins and your first cousins once removed. You share common great grandparents with the children of your parent’s first cousins, so they are your second cousins.
You may find this chart helpful.