Nuclear fusion is the process of two atoms colliding hard enough for them to merge, creating a heavier atom (example: two hydrogen atoms are fused, the result is a helium atom). The current nuclear power plants use nuclear fission, which is the opposite of nuclear fusion. It's the process of making a neutron collide with a fissile atom like Uranium 235, which causes it to split, releasing alot of energy and another neutron.
Normally, hydrogen atoms repel each other due to the Coulomb force. Nuclear fusors speed atoms up so fast that they collide with enough energy to bypass this force, and they get so close that their nuclear strong force pulls and binds them together, both become one atom.
This process releases energy, a neutron, and creates a heavier atom too.
Nuclear fusion is safer than nuclear fission, the technology that nuclear power plants these days use. Unlike fission, it doesn't create impossible to dispose of, dangerous nuclear waste.
Not only that, but it would be almost unlimited energy if someone can figure out how to harness the true energy of it. This is because the fuel can be made from water!