Meaning of the X-bar item in X-bar theory?

In X-Bar theory, what meaning do the X' nodes themselves represent?

I (believe I) understand the meaning of the XP items: they are the constituent syntactic category phrases. The constituents of a sentence that have syntactic function are XPs (is that correct?)

I also believe I understand the X items themselves, the heads of the XPs. It is the lexical item that gives the constituent it is in its syntactic category, based on its lexical category.

However, how can I understand what the X' nodes/items/subtrees are meant to represent?