Activity tells us how many nuclei in a radioactive source decay every second. It is measured in becquerels (Bq). The unit is the becquerel.
One becquerel is one decay per second. A source with an activity of 300 Bq has 300 nuclei decaying every second. A source with a higher activity in Bq is giving out more radiation each second, so it is more active than one with a lower activity.
You can use the unit to work out the number of decays over a time. A 50 Bq source gives 500 decays in 10 seconds, because 50 decays per second multiplied by 10 seconds is 500. A source of 1000 Bq is more active than one of 100 Bq.