Metrics
Frequency
Frequency is the average number of times each reached person was exposed to a campaign, calculated as impressions divided by reach. Too little and the message does not register; too much and the budget buys irritation and waste on people already saturated.
In practice
Frequency is a distribution, not just an average. An average of three can hide a long tail of people hit twenty times while half the audience saw the ad once.
Frequency capping and sequencing are the practical controls; the right level depends on message complexity, competitive noise and the length of the flight.