A euphemism is the substitution of a more favourable term for another expression. For example, 'adult entertainment' is a euphemism for 'pornography' and 'rest room' is a euphemism for 'toilet', even though one doesn't go there to 'rest'. (In fact, the word 'toilet' is a euphemism itself for a dressing room which also contained the appropriate bathroom feature. 'Wash room' is a more accurate alternative term.)
There's a tradition in the theatre where people refrain from uttering the name 'Macbeth', since that is a name of ill-omen. Instead, they use the euphemism 'The Scottish Play' and by doing so, fall victim of bowing to superstition.
Innocuous sounding euphemisms can usefully avoid causing offence, but they can also be abused. Euphemisms can sanitize things or conceal realities, which however harsh, should be exposed. Hitler's euphemism for genocide was 'final solution' and from the outset of the wars in Afghanistan and Iraq several notable examples appeared.
Saying a toilet is a place for resting is just silly, but using euphemisms to conceal the evils of war is obscene.