The Geohash algorithm takes a latitude/longitude pair and makes a base 32 string out of them. That is useful in itself, it packs the floats in a convenient structure for saving in a database, no need to worry about rounding or significant digits. For example, the geohash of the location of our offices (57.6997, 11.9661) is “u6280rcky”.
Geohashes also have a very useful property: close locations (can) have a common prefix. The reason for this is that geohashes are actually bounding boxes. Chop things off the end and you get a bigger bounding box.