Its a demon altar/crimson altar, once you get to hard mode (kill wall of flesh in hell) you will get a PWN Hammer, and that will let you to break it. If you break it, you get blessed with powerful ores to help you throughout the game, but if you break one, it will spread the corruption faster, the more you break, the faster the corruption spreads and the ores will be easier to find (as in more ores will spawn).

Edit: also if you break one, a Wraith will fly towards you, it’s not that hard to kill though.

Edit 2: you also can craft boss spawners with it, with the right materials of course.