Tritium has a half-life of 12.3 years. This means that after 12 years and four months, half of the atoms of the original amount of tritium are gone. This is due to radioactive decay of the element.
The atoms do not simply disappear. In the case of tritium, the atoms release a beta particle and an anti-neutrino as they decay. Beta particles are equivalent to electrons, and after losing one, the tritium atom becomes a helium atom. Thus, after 12 years and four months, half of all the atoms in the tritium have become helium. Tritium itself is considered a radioactive form of hydrogen and has similar properties to the usual form of hydrogen.