a group of words in a fixed order that has a particular meaning that is different from the meanings of each word on its own

To "bite off more than you can chew" is an idiom that means you have tried to do something which is too difficult for you.

the style of expression in writing, speech, or music that is typical of a particular period, person, or group

Both operas are very much in the modern idiom. 两部歌剧都很有现代风格。