Frequency is the average number of times each person saw your ad over a period, calculated as impressions divided by reach. On Meta it is the earliest warning that you are over-mining a fixed audience.
Reach versus frequency
Reach is how many unique people saw the ad; frequency is how many times, on average, each of them did. Frequency is impressions ÷ reach. A little repetition aids recall; too much burns goodwill and money on people who have already decided.
The saturation signal
When frequency climbs while results fall, you have reached the best-matching people in the audience and are now paying to show the same ad to the same people again. That is audience saturation, and the fix is a new audience or fresh creative, not more budget on the same ad set.