Basically radioactive substances have a half life.
The is a measure of how long it takes for them to decay by half (in simple terms)
When a substance is radioactive it is unstable and wants to become stable again.
So it will keep on decaying (emitting radiation) in order to become more stable
Say the radioactive substance has 20 radioactive nuclei to begin with
It decays (emits radiation) until there are only 10 radioactive nuclei (as this makes it more stable)
This takes 30 minutes
The half life of the radioactive substance is therefore 30 minutes.
Hope that helps. Feel free to ask any more questions.