Half Life time period exists for Radio active elements. When a radio active substance like Uranium is kept open, it radiates energy in terms of alpha rays etc. The atoms disintegrate slowly and disappear in to the space around. The mass of the substance decreases at a rate.
The time period during which half of the initial mass of the substance disintegrates and radiates, leaving behind just half of the material we stated with, is called the half life period,
Te definition can also be given in number of atoms of the substance.