Bedrock is an unbreakable block. It generates naturally with the world, appearing underneath the Main Door and covering the bottom of every world.
Trying to break the bedrock would display a message which reads: "It's too strong to break.". The only way to move bedrock is to use a Door Mover, which moves the main door and the single bedrock under it.
Breakable Bedrocks Bug
A bug in an update during the beta version of Growtopia allowed players to break bedrock, collect them and seeds. This has since been patched and most instances of the item have been removed. Any player who picks up Bedrock or a Bedrock Seed into their inventory will have it instantly removed from their backpack. However, a few overlooked instances remained in worlds, some of which have been put on display by their discoverers. In a blog by Seth he mentioned that he wouldn't be surprised to see a bedrock tree growing!
Any bedrock or bedrock seeds collected by players are now removed from the player's inventory instantly, but some worlds, like CARNAGE still have bedrock and bedrock seeds in them today. Developers may be able to move and break bedrock. Some SUPERMODDANCEPARTY worlds have random bedrock placed in them.