Command Block

Server admins place it, not players. Its own text says only that it runs commands, and the tables give it no recipe, no workbench and no research cost, so on a normal survival server you will neither craft one nor pull one out of a crate.

As far as players know, it lives in the toolbox of whoever runs the server. Power reaches it, and it executes a console command that was typed into it beforehand. That lets a custom map or an event server react to a button press: open the door to an arena, hand out a kit, teleport the winner.

So if you run into one on a community server, an admin built that contraption. For your own base, stay with the regular electrical parts and a wire tool. No survival circuit needs this block.