You find out that it takes 2 sec. for the swing to complete one cycle. what is the swing's period and frequency?

The time it takes "to complete one cycle" is a perfect definition for period.

So the 2 sec that you measured is the swing's period.

Frequency is just 1/period (the 'reciprocal' of the period).

For this swing, the frequency is 1 / (2sec) = 0.5 per second = 0.5 Hz.

("Hertz" means "per second")