What it is
Gross rating point (GRP) is a common metric in traditional ad buying that measures the exposure of a given ad campaign.
GRPs quantify impressions as a percentage of a target audience, multiplied by the frequency in which that audience sees the ad
How to use
GRPs measure in a standardized way how many times an advertisement is viewed, and thus allows for comparing media performance across campaigns, regions, or channels.
Additionally, GRPs allow for calculating overall media performance, adding up GRPs across campaigns, channels and time periods (e.g. weeks).
What to consider
The base for measuring reach in GRP calculations is typically the largest measured population with reasonable access to media.
Sometime the notion TRP (Target Group Point) is used as well. TRP and GRP measure the same thing, but TRPs is used for performance for a specified target audience.