This is just the number of times something happened (frequency) divided by the number of times you tried the experiment.

Relative Frequency = Frequency / Number Of Times Experiment Run

Example: A coin is tossed 80 times and land tails 10 times. What is the relative frequency of landing tails?

Relative frequency = 10/80 = 1/8

Note: You might then be asked if this suggests that the coin (or whatever it is in the question) is fair or biased. Clearly here the relative frequency should be around 1/2, so a relative frequency of 1/8 suggests the coin is biased.