So.

What does the Cosmic Key do?

It bypasses the normal realmgrid connection rules.

Every realm has 6 connection point. Forming a triangular grid. That is the realmgrid.

When the first person completes a realm's task, they can link to any adjacent unlinked realm.

Cosmic Keys allow you to connect between any two realms, regardless of claims or ownership. That link can then be used by every player forever.

Why is that useful? Couldn't I just walk around?

You could, but this game has only two fast travel locations.

Also, players intentionally block realms, and even entire rings, from other players and constellates.