An idiom is a group of words, like a phrase, where the meaning is usually figurative. A few examples of idioms are listed below to help you understand what they are:
(1) Idiom: 'A piece of cake.' This does not refer to a literal piece of cake, it means that something is easy to do.
Example: One chore people do to keep their house clean is vacuuming. A chore like that is probably easy for you to do, right? In other words, vacuuming the house is 'a piece of cake' for you.
(2) Idiom: 'A fish out of water.' This, of course, does not refer to a literal fish. Rather, it refers to people who are in uncomfortable or unfamiliar situations. People in such situations are like a fish that's out of the water.
Example: Imagine someone has never been to the gym before, but decided to go for the first time. They would be unfamiliar with a lot of the exercise equipment there. Thus, the person is like 'a fish out of water.'
(3) Idiom: 'making a mountain out of a molehill.' This means that someone is taking a small issue and making it into a bigger problem.
Example: Two people are talking, and one of them accidentally says something that the other finds offensive. The person apologizes, but the person who took offense starts yelling loudly. Well, this angry person took a small issue and turned it into a bigger problem, or in other words, he made a mountain out of a molehill.
The above are three idiom examples, and as you can see, their meanings are not immediately obvious just by looking at them. So if you come across an an expression that you're not familiar with, then this website can help. How so? Well, we have an alphabetical list that contains plenty of phrases with their meanings included, so you might find that to be helpful. There's also more examples of idioms all over this site if that's what you are looking for. If you can't find a specific one, then check back another time because new ones will be added.