Pi mesons, also known as pions, are subatomic particles that play a crucial role in the strong nuclear force, which holds atomic nuclei together. They are classified as mesons, meaning they are made up of a quark and an antiquark. There are three types of pions: positively charged (π+), negatively charged (π-), and neutral (π0). Pions are relatively light compared to other mesons and have a short lifespan, decaying into other particles. They are essential in mediating interactions between nucleons, such as protons and neutrons, within atomic nuclei, contributing to the stability of matter in the universe.