When the atoms of radioactive elements disintegrate, they form new elements with the emission of alpha and beta particles. The rate at which atoms disintegrate depends on how much radioactive element is present. As less and less radioactive isotope is left, the disintegration becomes slower and slower, but it ia never complete. The total life of a radioactive element is infinite, so the rate of radioactive decay of an element is described by its halflife.
HalfLife is a measured value for the amount of time it takes for a substance to decay to half of its original value. It can also be abbreviated by t_{1⁄2}. It happens when a radioactive isotope goes through decay, which constantly halves the value in order to make it more stable. To better understand let's take an example. Halflife of radium is 1600 years which means that a given sample of radium will disintegrate half in 1600 years. If we start with 1 gram of radium now, then 0.5 gram of it will have disintegrated in 1600 years. After 1600 years, half of what will be left will disintegrate to half and so on.
Different radioactive elements have different halflife periods.
Element  Radioisotope  Halflife

Polonium  Po214  1.5 × 10^{4} seconds 
Iodine  I131  8 days 
Phosphorus  P32  14.3 days 
Iron  Fe59  44.3 days 
Cobalt  Co60  5.3 years 
Radium  Ra220  1.59 × 10³ years 
Carbon  C14  5.8 × 10³ years 
Uranium  U238  4.5 × 10^{9} years 