A URL scheme is a way for other apps or just links in a mail to wake up another app and ask it to do something. So I could send you cachly://gccode=GC70 and you could click that and have Cachly open up to the oldest multicache in the world.
Continuing with that line of thought, I have made a feature request to let Cachly users define their own "URL bookmarks" that would let Cachly integrate with other apps or web services. In Geosphere, I have a bookmark I defined that opens the cache location in Google Street View. Super useful.