An idiom is an expression whose meaning is difficult or sometimes impossible to guess by looking at the meanings of the individual words it contains.Since idioms are used so extensively in speech and writing,one should learn them to avoid misinterpreting the information you hear or read.
We can also say that an idiom (also called idiomatic expression) is an expression, word, or phrase that has a figurative meaning conventionally understood by native speakers. This meaning is different from the literal meaning of the idiom's individual elements. In other words, idioms don't mean exactly what the words say.
Examples of Idiomatic Experssions are;
1. Its raining cats and dogs - It means that it is raining heavily.
2. Kill 2 birds with one stone - It means getting 2 things done at the saame time.
3. Let the cat out of the bag - It means It means to let the secret out.