Along with protons, neutrons make up atomic nuclei, forming the foundation of all matter. Unlike protons, which carry a positive charge, neutrons have no electric charge at all. That neutrality is what lets them help hold atomic nuclei together by balancing the repulsive forces between positively charged protons, all thanks to the strong nuclear force (transmitted by gluons).
Inside an atom, neutrons are stable and can exist indefinitely, but on their own they don’t last. A free neutron gets about 15 minutes before decaying into a proton, an electron, and an antineutrino. This process (beta decay) is an important player in nuclear physics, responsible for everything from radioactive transformations to the nuclear reactions that power stars.