UI kit is a set of all the elements on which the UI of your site is built (UI elements). In essence, it is a visualization of each of the elements at any stage of the user’s contact with the interface.
Example of a part of UI kit
Basis
Atoms
Tip: it’s better not to create a bunch of icons with different sizes. If one size is not enough, then ideally select several sizes and follow some system. For example, let all icons for buttons be 16px, and icons in tabs — 32px.
Molecules (various composite blocks of atoms)
Organisms