The word geohash is both a noun and a verb, used in a few different ways.
1. noun: A geohash is a random coordinate generated in every graticule across the world every day by the Algorithm described in XKCD comic #426. These coordinates can be used as destinations for adventures, à la Geocaching. They can also be used for local meetups. Please refer to the Main Page of this wiki for more information.
2. verb, intransitive: to geohash is to embark on an expedition the purpose of which is to reach a geohash and/or meet others attempting to do the same. Synonymous with go geohashing.
3. verb, transitive: to geohash <something> (i.e. a graticule, country, large region or continent) is to attempt to reach geohashes in that jurisdiction. Also used with a preposition "geohash in." For smaller areas, (e.g. cities, parks, neighbourhoods) the preposition is always used.
For a little more interactive discussion consider joining the #geohashing IRC channel on Foonetic.