The eraser is an item used as a tool in Debug Mode. It is used to remove all blocks (sets all blocks to air) in an area previously selected by the Cursor, including any blocks added by AdventureCraft.
If you wish to undo an erasure, hit Ctrl+Z (Command+Z). Be careful, though, erasing large areas, undoing with Crtl+Z, and even re-doing with Ctrl+Y may cause your game to crash.
Eraser Hotbar Icon
An area selected by the Cursor, ready to be erased
The area, previously selected, now fully erased