A euphemism is nice, polite way of saying or describing something when it would not be appropriate to use the more common - more abrasive word for a situation that has the potential to be embarrassing to someone.
Ex.
'passed gas' vs. 'farted'
passed away vs. died
'large' vs. 'fat'
The word is connected to connotative meaning in that the connotation of words has to do with appealing to one's emotions, feelings, perceptions, interpretations in meaning, especially with advertisers or swindlers, for example. In other words, the connection is like you might not want to offend your wife by calling her mother fat, so you instead call her large. In the middle of a church service, you wouldn't want to say the word 'farted' to the clergyman sitting next to you. etc.....