Geospatial queries allow you to find documents that are tagged with longitude / latitude coordinates by distance from a centre point.
ForerunnerDB will return results ordered by distance from the centre point, closest first. Geospatial indexing in ForerunnerDB takes the longitude and latitude coordinates in a document and converts them to an internal geohash. Geohashes are then used inside a b-tree to query nearby hashes in a performant manor.
Geospatial queries will only work on collections with a geospatial (2d) index.
You can read the ForerunnerDB manual to find out more about geo-spatial queries here: https://github.com/Irrelon/ForerunnerDB/tree/master#geospatial-2d-queries