Okay kiddo, so imagine you have a big family tree with lots of branches and leaves. Your parents, grandparents, and siblings are all on the same big branch as you because you are all related in a direct line.
But what about your aunt, uncle, or cousin who is not on the same branch as you? They are still related to you, but in a different way. They are your collateral relatives or collateral descendants.
Collateral means something that is not directly connected or in a line. Descendant means someone who comes after or is born after someone else.
So, a collateral descendant is someone who is related to you but not in a direct line. For example, your cousin's child or your grandparent's sibling's grandchild is your collateral descendant.
It can be a little confusing, but just remember that collateral descendants are still part of your big family tree, just on a different branch than you.