Tree Select is a choice menu arranged like a set of folders. Some choices contain smaller choices, which a person can open or close before selecting one. It is like picking a folder from a filing cabinet when the folders are nested inside one another.
For example, an organization form might ask someone to choose a team: first open a department, then select one team within it. The nested layout helps when options have a clear parent-and-child relationship.
Use a flat list when all choices are on the same level. A tree can be confusing if people do not understand why some options sit inside others, so make the grouping names familiar.