c:geo will register itself for geocaching specific hyperlinks, so that c:geo will be shown as possible target app, when you tap on one of those links in another app.
For example c:geo will register for links starting with
http://coord.info/GC which is used in mails sent by geocaching.com to their users and target a certain cache listing. Tapping on this link in any app and selecting c:geo as target app, will directly open the cache details of the linked cache in c:geo. The same applies for geocaching related links of other geocaching platforms.
When being asked which app to use to open these links, you can also select c:geo as default app for this link type. Afterwards your device will directly start c:geo when such a link is tapped.
In Android 12 Google changed the functionality of web-links. See this FAQ-entry for details how to enable this feature on Android 12.