When one is told not to say certain words, it's probably because they're vulgar. But that's not always so. These prohibited words couldn't be more normal, but they're just not proper enough for the Royal Family.
For example, the word "pardon" is forbidden. You have to use "excuse me" or "sorry". One would wonder which context would ever require the use of the word "toilet" in front of the Queen, but this is also prohibited because it is of French origin - instead you should use the euphemism "loo". Also, when one speaks with the Queen, one doesn't say "perfume" but "scent". And one doesn't use the word "tea" for the evening meal but either "dinner" or "supper".
After having "dinner" or "supper", you will probably go through to your "living room", another word that the Queen doesn't want to hear. Around her it should be called the "drawing room", literally "a room to withdraw to".
Children also have to comply with the protocol. The words "mum" and "dad" are forbidden, so Charles, Anne, Andrew and Edward must call their mother "mummy"...and nothing else!
Take a look at the video above for the full list of words that are banned around Her Majesty!