The five essentials are a checklist to make sure you are sailing your boat with maximum efficiency. Here is a video, made by the RYA, explaining the five essentials and featuring Olympic Medalist, Shirley Robertson.
Here is an easy way to remember them suggested by Sam Waites, just ask yourself the question "Can this boat sail better?" and use the first letter of each word as a reminder
|Can||C||Course made good|
|Better?||B||Board (daggerboard, or centreboard)|