Half life of an element refers to the time period over which half of the original amount of element will degrade and we will be left with only 50% of the original amount. For example, if the half life of an element is 12 hours and we start with an amount of 100 g; then after 12 hours, only 50 g of the element would be left. After another 12 hours, that is 2 half lives, only 25 g of element would be left.

The half life of an element is used to determine the amount of element we will be left with after the end of a certain time period. For example, if we are planning to dispose off a harmful element that has a half life of say 10 years. If we have a total of 100 kg of the material, we will only have to worry about 50 kg after 10 years from now and so on. 

Half life is also used in dating of fossils and waste management planning, etc.

Hope this helps.