A basemap provides a background of geographical context for the content you wish to display on the map. When you create a new theme you can choose which basemap you wish to use.
You can change the basemap at any time by selecting one from the tabs, or by using a custom layer as the basemap.
There are several basemaps available for use, including satellite imagery and WMS.
To change the basemap, click on the tab in the top-right corner of the map area and select the basemap you want to use.