WTF is an "Atom" in X11 ?

https://tronche.com/gui/x/xlib/window-information/properties-and-atoms.html

A property is a collection of named, typed data.

so, like, a structure or something???

For each named property, a unique identifier (atom) is associated with it

so, like, an alias? A #define?

Is this something really complicated or are they just renaming known C terms and concepts to make them special and confusing?

What the heck is an atom and why do I need it?