A common feature of many digital circuits is a high-frequency clock
with a square wave output.
If this signal of frequency f drives the clock input of a JKFF wired
to toggle on each trigger, the output of the flip-flop will be a
square wave of frequency f/2.
This single flip-flop is a divide-by-2 counter.
In a similar manner an n flip-flop binary counter will yield an
output frequency that is f divided by 
.