A trade plan is a checklist that you should follow during any day of trading. It acts as a last line of defence against impromptu actions. Your trade plan is likely to be an overview of the actions contained within your trading journal.
They should always contain general rules that you must adhere to. They can also be more specific and tell you to execute an order when a specific price is hit.
Whichever approach you choose the role of the trade plan is to stop you from making rash decisions whilst the market is open.