This is a tool that can limit the amount of times the same person can be served your Display or Video ads across a given time period. The capping works differently with Display and Video campaigns. Display campaign ads can be limited according to campaign, ad group or ad, while Video can only be limited by campaign. If a video is reused across campaigns, each view will count toward the frequency cap of each campaign.
PPC Management Agency Google Adwords Metrics Frequency capping