The strength of a radioactive source gets weaker as time goes by. Every time the source emits a particle its activity decreases.
There will be fewer radioactive atoms and more stable ones.
The average time taken for the activity of a sample of radioactive material to drop to HALF its original value is called the HALF LIFE of the source (written as T or T1/2 ).
This half-life depends only on the material of the source and different radioactive isotopes have different half lives.
This means that if we have some material that has a half life of 20 days then after 20 days it will only be HALF as radioactive as it was at the start, after another 20 days, i.e 40 days after the start it will only be a quarter as radioactive, one eight after 60 days and so on.