Subatomic means smaller than an atom, or belonging to the inside of an atom. It is a physics word, and it almost always appears in the phrase subatomic particle — electrons, protons, neutrons, quarks, and the rest of the very small things that atoms are built from or break into.

For a long time people thought the atom was the smallest possible piece of matter; that is exactly what its Greek name says. Then physicists found pieces inside it, and needed a word for them — hence subatomic. So the word carries a small joke in its history: it names the parts of the thing that was supposed to have no parts.

Outside science, English speakers use it loosely for anything unbelievably tiny, usually with a smile: "the portions were subatomic", "my chances are subatomic". That is informal exaggeration, not a real measurement. The word is an adjective only — there is no verb or common noun form — and it is used before nouns (subatomic physics, subatomic scale) or after be (the differences are subatomic, in the joking sense).