The activity of a radioactive source is the number of nuclei that decay each second. Its unit is the becquerel, symbol Bq. The unit is named after Henri Becquerel, who discovered radioactivity.
One becquerel is one decay per second. A source with an activity of 200 Bq has 200 nuclei decaying every second. A source with an activity of 5000 Bq has 5000 decays every second, so it is more active than the 200 Bq source. Activity in becquerels is the number of decays per second.
You can work out an activity by dividing the number of decays by the time in seconds. If a source gives 3000 decays in 10 seconds, its activity is 3000 divided by 10, which is 300 Bq. If it gives 600 decays in one minute, that is 600 divided by 60 seconds, so its activity is 10 Bq. Activity is measured in becquerels.