Bedrock (Also known as Adminium) is an indestructible block that makes up the underground borders. It cannot be mined nor placed by regular players.

Bedrock is used at spawn to create a platform for players to spawn on that is indestructible to griefers, without actually having to protect the spawn.

There also exists a technical block called Invisible Bedrock which is an invisible but collidable block that is used to prevent players from going out of bounds where regular bedrock doesn't exist.