A euphemism is a word or phrase that is intended to be more polite than the word it is replacing, for example:

Make love is a euphemism for having sex.

Use the restroom is a euphemism for going to the bathroom (which is itself a euphemism for urinating or defecating).

Pass away is a euphemism for dying.

Many euphemisms are related to bodily functions (going to the bathroom, sex, etc.) and death, although they can be about any unpleasant or uncomfortable topic:

Unfortunately, because of her consistent lateness, we had to let the new secretary go. (let go is a euphemism for firing someone)

Idioms are often also used in politics:

Friendly fire is a euphemism for soldiers killed accidentally by their own army.

Collateral damage is a euphemism for innocent people who aren’t soldiers being killed in a war.

One of the reasons euphemisms are hard for ESL students to learn is that they are very often idiomatic expressions. On the other hand, euphemisms can be very interesting, as they are very cultural. What may be OK to talk about in your culture, may be something that is only talked about using euphemisms in another culture.