A custom POI is a location you enter or load into your GPS. On many GPS models, they’re managed differently than waypoints and usually have special features associated with them (such as proximity alerts, speed/proximity alerts, etc.).
One of the more common uses for custom POIs is to warn of nearby “red light cameras”, “safety cameras”, or “speed traps”. Custom POIs are also commonly used for sets of location data provided by 3rd party companies to identify their store locations, RV parks, etc.