If you have a “hole” in your map then it is possible to create a polygon to “fill” the hole by drawing it freehand, but this is not always easy to do.
Instead, it is possible to use adjacent map polygons (e.g fields, woodland, etc) to create a new polygon which precisely matches the “hole” in the map. The video below shows how this can be done.
Note: To be able to do this, you must have adjacent polygons which completely surround the hole in your map.