Commands are orders given to your Townsfolk via the bar at the bottom of the screen. They allow for quick mass tasks to be issued. When an order is selected to be issued, be it mining, placing a stockpile or putting down a storage barrel, the order will appear on the top right hand side of the screen and labeled your order.
Digging is almost the same command as mining but Townfolk will dig one block below the current level. Right clicking and selecting "Dig ladder" will first dig a hole and then replace it with a set of stairs. Dig commands are outlined green.
Mining allows your Townsfolk to remove adjacent tiles and harvest the minerals within. Mud can be mined from dirt, iron ore from iron tiles, etc. The order is issued on blocks of the same level and is outlined yellow.
Delete commands can only be issued on existing scaffolding. If used on the lowest scaffold, the others above will succumb to gravity and fall downwards, allowing you to delete them all without changing levels.
Cancelling issued commands is as easy as using the Cancel command and dragging the box over any tiles you've previously issued commands on. Townsfolk will still come if they're already on the way, but once they get to the tile, they will ignore it and take up a new action. Commands like digging and tilling can be cancelled even if the townsfolk has started the task.