Microprocessors use a clock to synchronize the different circuits within them. The clock circuit generates a waveform (usually square). The signal stays high and the falls low.
The clock period is the time which begins when the signal becomes high, then falls low and then becomes high again.
For instance, a 33% duty cycle implies that the signal will be high for 33% of the clock period.
The duty cycle can be expressed as follows
Duty Cycle = (Pulse Width/Clock Period) * 100