This is text from NAV help (you can find many answers in help, msdn and offical MS training materials):
You use the planning worksheet to calculate and adjust an item replenishment plan. You can enter items in the worksheet in the following ways:
-Enter the items manually in the worksheet and fill in the relevant fields.
-Use the Calculate Plan batch job. This calculates a replenishment plan for items and stockkeeping units that have been set up with a requisition system of Purchase, Prod. Order, or Transfer. Note that you can calculate either a net change plan or a regenerative plan. When you use this batch job, the program automatically fills in the Action Message field with a suggestion for an action you can take to replenish the item. This could be increasing the item quantity on an existing order or creating a new order, for example.
Planning worksheet lines contain detailed information about the items that need to be replenished and the corresponding order proposals that the program has made. You can edit and delete the lines to adjust your replenishment plan, and you can further process the lines by using the Carry Out Action Message batch job.
For details about planning with locations and transfers, see More Information.
Warning Messages and Red Text:
Any planning line created for an unusual situation will display a warning icon in the Warning field, which you can click to read additional information.
On such lines, the Accept Action Message field is by default not checked because the lines require special attention before they can be accepted.
Apart from the different warnings, planning lines may include red text in two situations:
-If a planning line suggests a starting time earlier than the work date, the value in the Starting Date-Time field is in red text.
-If a planning line suggests to change a released purchase or production order, the value in Ref. Order No. field is in red text.