Answer:
A mnemonic is a memorable phrase that makes it easy to recall lists using the initial letter from each word.
For example the colours of the rainbow can be remembered using the mnemonic 'Richard Of York Gave Battle In Vain'.
The trick is to take the initial letter from each word (ROYGBIV) and that should be enough to help us remember that rainbows are made up of the colours Red Orange Yellow Green Indigo Violet (ROYGBIV).