A Plan is a Blank Plan that has a crafting recipe recorded on it.
Plans are written on a Project Table, and are used to store crafting recipes. When a Plan is placed in the plan slot of a Project Table, the crafting recipe will be displayed in the background of the crafting grid. Any materials not present in the Project table's inventory will be highlighted in red. Using Plans, you can craft an item without placing anything in the crafting grid, provided all the materials are present in the Project Table's inventory.
This is especially helpful for some GregTech devices that require multiple levels of crafted items (for example, the Industrial Grinder).