It's the time it takes for half of the atom to have decayed.
For example: atom X has a half life of 1 year. After 1 years, half of it will have decayed leaving only 1/2 of it. After 2 years, half of what's left (1/2) will have decayed leaving only 1/4 of the original atom left. After 3 years, only 1/8 of the original atom will be left. 1/16 after 4 years, 1/32 after 5 years, etc.
the amount of time it takes for the radioactive atom to decay by half. the decaying activity is measured in becquerels. e.g te half life of an atom is 10 hours therefore in 20 hours 1/4 of the atom hasn't decayed.