Sub-: A Latin prefix meaning "under," "below," or "smaller than." (Think of "submarine" or "subway").
Atomic: Relating to the atom.
So, "subatomic" describes anything that is smaller than an atom or found inside it.
For a long time, scientists thought the atom was the smallest possible thing. We now know that atoms are actually built from even smaller pieces. These pieces are called subatomic particles. The three main ones are protons, neutrons, and electrons.
Understanding "subatomic" means understanding the hierarchy of scale.
Things you can see with your eyes (rocks, water, people). These are made of Compounds and Molecules.
The individual building blocks of matter (Carbon atom, Oxygen atom). They define the elements.
The parts inside the atom. This is where the rules of physics get weird (Quantum Mechanics). Protons and Neutrons live here.
We can go even deeper! Protons and Neutrons are made of even smaller particles called Quarks. Electrons and Quarks are currently thought to be the smallest possible things.
Deep underground in Europe lies the Large Hadron Collider (LHC), the biggest machine ever built. Its job is to study the subatomic world. Scientists speed up beams of protons until they are moving almost at the speed of light, and then crash them into each other.
Why smash them? By breaking these particles apart with incredible energy, scientists can briefly create and observe rare subatomic particles that haven't existed naturally since the Big Bang. This helps physicists answer fundamental questions about how the universe works, such as "What gives matter mass?" and "What is dark matter?" It turns out that the tiniest subatomic pieces hold the biggest secrets of the galaxy.