This sets up an invisible rectangular block on the screen with top-left co-ordinates X1,Y1 and bottom-right co-ordinates X2,Y2. Objects are not allowed to cross the borders of this block (unless they are told to ignore blocks). The block can be removed using the unblock command.
Note: You can only have one block on the screen at a time. If a block already exists, and you use the block command again, the new block will be there but the old one will be gone.