A basemap is a basic set of map data that’s built into a GPS device and cannot be removed.
Basemaps usually only contain major roads and landmarks. In such cases, they are intended to be used for areas where you haven’t loaded detailed map data into your GPS unit’s memory.
The specific information included in a basemap varies, but you'll usually find highways, larger roads, cities and towns.
This example screen from a Garmin StreetPilot shows the level of detail available in a typical base map. It shows highways, interstates and major roadways. Smaller local roads are not included in this base map.