This happens when you're adding a certain type of value to a built-in list that expects certain other types of values. For example, atom.contents and client.screen can only contain movable atoms (except area.contents). Other types of values, such as strings, numbers, type paths, and null, will cause an error when added to these lists. Other restrictive lists include client.images and atom.overlays. Pretty much every built-in list is a special list with its own restrictions.

Basically, it means exactly what it says.