Frequency & Period are inverse of each other.
It is indicated by the following formula:
Example1. A wave has a frequency of 100hz. Its period(T) is given by T = 1/ F = 1/ 100 = 0.01 sec
Example2. A wave completes its one cycle in 0.25 seconds. Its frequency is given by F = 1 / T = 1 / 0.25 = 4 Hz