Placing Ocuvigil next to Bee Queen's hive will keep the players informed whenever she respawns. There are multiple similar uses of this structure such as keeping an eye on the spider dens, the Ruins awaiting to respawn.
It's referred to as a "sentryward" in the game files. Also, a similar structure with the same name and function exists in multiple MOBAs.
The name "Ocuvigil" is a portmanteau of "oculus" and "vigil".
It used to be crafted with a Yellow Moonlens, that was replaced by a Purple Moonlens in a later update, as Yellow Gems are not easily available in worlds without Caves.
Placing Ocuvigil on a boat that's been moved with an oar used to result in a slow and random, but automatic exploration of the ocean. Creating such set would've cost 6 planks, compass and a purple moonrock. As of the Waterlogged Update, this can no longer happen, since boats now no longer retain their momentum after rowing only once.