Idioms are forms of figurative language that contain a separate meaning from the words being used. For example, “The lights are on, but nobody is home.” does not mean that there is a literal lit up house with no residents. It actually means that someone isn’t very smart, or possibly brain-dead.
Some examples would be “kick the bucket”. A person isn’t going to actually be kicking a bucket. It means they’re going to die. If you say “Our old cow finally kicked the bucket,” it just meant that the cow died. If you’re “bent out of shape”, you’re not actually bent a funny way. It means that you are upset or worried about something. If you hear something or hear from someone “out of the blue”, it’s something that comes about unexpectedly. If you said something was “a piece of cake”, it would mean that it was easy to accomplish. It’s as easy as eating a piece of cake.
If you buy a “lemon”, it means you buy something that breaks immediately. A lot of times this refers to a vehicle. If you buy a used car and you think, “Oh, I’ve got this great deal,” and then it breaks within a few days, or one of the parts breaks, then people say, “You really bought a lemon.” It could be anything. If you bought a sofa and you got home, you had it for a couple days, and you realize that one of the cushions was really stained or the middle of it was missing springs so you kind of sunk into it, then you might have bought a lemon in that way. It basically means buying something that breaks immediately and wasn’t quite what you thought it was. It turned out worse.
My last example is “hit the hay”. You’re not actually hitting a bale of hay. It just means you’re going to bed. Idioms are figurative language. There are lots of different kinds of figurative language that you can use, but this is a fun way to add color to your writing. There are countless idioms, and most of them are commonly known. If you use them in your writing, your reader will know what you mean, but they’ll like that you tweaked it a little and made it a little more clever. You made it not just writing out the literal phrase. Especially if you’re repeating a certain kind of topic, or certain details several times, you may to find different ways to say it. Sometimes an idiom can do that for you.