The period ( measured in s) is the amount of time is takes for the pendulum to make a single cycle, i.e. how long it takes to swing in both direction. The frequency (measured in Hz) is the inverse of the period, the number of cycles it completes in one second.
So long as you know one you can find the other by diving 1 by it. Period = 1 / frequency, and frequency = 1 / period
If the pendulum takes 2s in each direction, then it has a period of 4s. So the frequency = 1 / 4s = 0.25Hz
