Frequency capping is an act of limiting the number of times that one user will see the ad during a given period of time. If a frequency is too low, users are unlikely to remember your brand to the needed extent. If a frequency is too high, users become annoyed because they are forced to see the same content too often.
Marketers often choose between higher frequency (fewer people more frequently) and higher reach (more people less frequently). Higher frequency is mostly chosen by small or new brands since they want to raise brand awareness. The cold audience of such businesses will ignore the ad if they see it only once. Higher reach is chosen by well-known brands whose aim is to sell to more people.