Atoms never rest: the hotter they are, the faster they move. In the Sun's core, where temperatures reach 15,000,000 °C, hydrogen atoms are in a constant state of agitation. As they collide at very high speeds, the natural electrostatic repulsion that exists between the positive charges of their nuclei is overcome and the atoms fuse. The fusion of light hydrogen atoms produces a heavier element, helium.
The most efficient fusion reaction in the laboratory setting is the reaction between two hydrogen isotopes deuterium (D) and tritium (T). The fusion of these light hydrogen atoms produces a heavier element, helium, and one neutron.