What Are Idioms and What Do I Tell Students?
An idiom is a phrase or expression with a figurative meaning that is different from its literal meaning. Idioms are common in everyday language, but they are hard to understand when you first hear them. For example, "under the weather" is an idiom that means "to feel a little sick."
The above explanation isn’t especially complex, but I wouldn’t use it with ESL students.
As mentioned in Twenty-Four Common Idioms with Meanings and Examples, students of English as a second language ought to start learning idioms from a low level. It makes sense that they build up their personal library of idioms as they go, bit by bit.
For beginners, I use a very common example, and point out how the meaning is hard to guess even if you know the meaning of each word.
In English, "a piece of cake" means a piece of cake, but it can also mean "something very easy." For example, "the driving test was a piece of cake." This kind of expression is called an idiom.
Sometimes students confuse idioms with proverbs (sayings). Point out that idioms are ways of saying regular things as opposed to expressing wisdom or inspiration.
For higher level students, I tell them it can be hard to guess the meaning of an idiom just from its words. However, in some cases, you can see a logic to the idiom that might help you to remember its real meaning.
You might be confused the first time that you hear, “I paid an arm and a leg for my new car.” If you think about it, giving your arm and leg would be a lot to give, much more than money. "An arm and a leg" means a high cost.
Hopefully, reflecting on idioms like this will make some of them easier to remember. For the most part, students will have to memorize the meanings of common idioms or rely on the context to provide them with clues.
There is a more detailed explanation of idioms over at Grammarly.
Using vs Understanding
It’s important that ESL students understand idioms, but using them is another story.
Idioms, unlike proverbs, only make up parts of sentences. Connecting them to other words often leads to mistakes and can really bring down a student's accuracy. For this reason, I don’t encourage students to use them often, but I also don’t discourage experimentation - unless, and I’ve seen this, they are trying to use them far too often.
For the best results, I just correct clearly wrong uses of idioms. I also say that understanding them is important, but using them is not the key to speaking like a native speaker.
Have students guess the meaning of several interesting idioms. In a classroom, you could have students do this in groups and then report back to the class.
Have students explain an idiom from their native language. They would have to explain the literal meaning and then the figurative meaning all in English. Unless they are high level students, they will probably need some time to plan what they are going to say.
A gap-fill involving idioms could help with accuracy, in particular if students need to make changes to the idioms for things like subject-verb agreement or tense.