A tree is a list arranged in levels, where one item can contain smaller items beneath it. It is like a family tree or a set of folders: a larger group can open to show the things inside.
For example, a file browser may show “Photos” with smaller folders for each year. Opening a folder reveals its contents, while closing it hides them again.
Use a tree when the items truly belong in nested groups. Give each group a clear name, and help people tell which parts are open and which item they have selected.