Reach is a percentage that measures the accumulation of audience exposure over time. Basically, the total number of people exposed to your communication. It counts the unique percentage of people exposed.
GRP—Gross Rating Points
Gross Rating Points measure the total amount of exposure of your communication. An example would be if 24% of househols had a particular baseball game on and you ran your ad three times during that game, you would multiply 3x22, giving you a GRP of 66%. GRP measures the total exposure.
Frequency is the ratio of GRP over reach. Frequency measures the repetition. The formula for Frequency is:
Frequency= Gross Rating Points/Reach