A period (T) is the time it takes for one complete cycle of a repeating motion — one full step, one full wingbeat, one full swing.

Frequency (f) is simply how many of those cycles happen per second: f = 1 / T. If a step takes half a second, its frequency is 2 steps per second.

Frequency is measured in Hertz (Hz) — one Hz means one complete cycle every second. A slow, heavy motion has a long period and a low frequency; a quick, light motion has a short period and a high frequency.