Hi & welcome,

As others have mentioned, it's a way to link to a community. More specifically, it's what allow to share a link to a community (not to a specific post, just the community itself) that will work no matter what instance anyone is clicking it from, unlike your usual link.

The Fediverse is still quite new and still has many rough edges. Its fragmentation into multiple instances, that are working together while also being completely independent is both one of its strength (independence, resilience and... choice) but is also one of those rough edges I mentioned: standard links don't work well when one wants to share a link between different instances. This is the best workaround for now ;)

edit: typos