Idioms are sayings, figures of speech or groups of words that have a very specific meaning. A colloquialism is an informal word or expression that is more used in speech rather than writing. A figure of speech is an informal word or phrase that is not to be taken literally and that is also more commonly used in conversation than in written work.
Generally speaking, it is not possible to understand the meaning of idioms, colloquialisms or figures of speech simply by reading the individual words. In most cases, idioms etc. are only properly understood by native speakers and cannot be translated into other languages. However, other languages may have similar expressions.
The advantage of using idioms, figures of speech and colloquialisms is that their meanings are very precise and they usually create vivid pictures in the mind of the reader or listener. This means they make language vibrant and colourful and they are very effective at conveying a specific idea, concept or image.