In an experiment
two dice are rolled 600 times and their scores added together each
time. A record is kept of the number of times the total is 2,3,4,...12.
The number of times a given total occurs divided by 600 is its relative
frequency. So, if a total of 7 came up 104 times then its relative
frequency would be 104/600.

In general, the relative frequency of
an event happening is the number of times the event happens divided
by the number of trials made. It is the fractions of the occurrences.