The Strong Nuclear Force
The Strong Nuclear Force is the strongest of the four fundamental forces. However, it also has the shortest range, meaning that particles must be extremely close before its effects are felt. It is the strong force that holds together the subatomic particles of the nucleus (protons, which carry a positive charge, so repel each other, and neutrons, which carry no charge.
The protons repel each other, being of like charge, and this force of repulsion is overcome by the strong nuclear force, created between nucleons by the exchange of particles called mesons.
The nucleons must be extremely close together in order for this exchange to happen, because the mesons are very heavy, so have a short range according to the uncertainty principle. The distance over which the strong force operates is about the size of a proton or a neutron. If a proton or neutron can get closer than this distance to another nucleon, the exchange of mesons can occur, and the nucleons will experience a net attractive force.