A gluon is the force carrying particle behind the strong force, the fundamental interaction that binds quarks together to form protons, neutrons, and other particles built from quarks. Without gluons holding quarks in place, ordinary atomic nuclei could never exist.
Gluons are unusual among force carriers because they themselves carry the very charge they transmit, called color charge. That means gluons can interact with other gluons directly, which makes the strong force behave very differently from electromagnetism at both very short and very long distances.
This self interaction is also why quarks and gluons are never observed alone in nature, a phenomenon called confinement, they only ever appear bound up together inside larger particles. Astrinova’s particle physics guide goes deeper into how the strong force works.