Planning was added into the game in Alpha 8 and it allows you to make visual walls, objects, and pathways without actually getting your workmen to build it.
It is useful for planning your prison to make sure that there is enough space before you make the change. Planning can be found near the bottom right of the interface.
Planning is free, and is already unlocked at the beginning of the game. To remove planning, you can either right click, or drag right click over the plans. Planning will remain there forever, until the planned object is placed there, hence removed.