As we know that radioactive elements continuously emit radiations. As a result they get transformed into new elements.
The elements emitting radiations is known as parent element and the element formed as a result of the emission of radiation is called daughter element.
The time interval in which half of the atoms of the parent element in any given sample decay into daughter elements is known as the half life of the parent element.
Thus if we start with a sample of 100,000 atoms of a radioactive element with a half life of T (e.g. 5 days), then after a lapse of T time (5 days), 50,000 atoms of the element will decay into daughter element.
After the lapse of another period of time T(5 days) -"after total 10 days"-, the number of atoms of parent element will reduce to 25,000 and so on.
Different radioactive materials have different half lives which may range from thousands of years to a fraction of second.