Reach and Frequency
Reach refers to the number of unique people (or in some cases unique devices or households) who were served an advertisement during a given time.
Frequency refers to the number of times the same individual (or again, device/household) viewed the same advertisement. This is usually reported as an average.
So, if a campaign achieved a reach of 50,000 and frequency of 3, that means that 50,000 unique people saw the campaign an average of 3 times during the reporting window.