A base map is an under lying map, usually calibrated to latitude and longitude, on top of which your custom map image sits. They usually display streetmaps or satellite views.
Common examples are Google Maps, Apple Maps (on iOS devices and Macs), Open Street Map, HERE Maps, Map Quest, and many others. There are also specialised mapping and Geospatial Information Systems (GIS) like ESRI/ArcGIS and tools like Mapbox. All of those can provide base maps, and moat of them offer other features or services like navigation, satellite view, topographic view, and many others.
Mapiddiction uses Open Street Map as its base maps.
