Any particular words you don’t know? Probably the most likely ones are para- and monophyletic. For a taxon (scientific grouping) to be valid, it needs to be monophyletic, meaning it contains the most recent common ancestor of the group’s other members and all known descendants of that common ancestor. Paraphyletic, by contrast, means not all the descendants are in there. For example, imagine if the mammals just randomly excluded the bears – that would be paraphyletic, because the bears also share a common ancestor with the other mammals.