An idiom is an expression, word, or phrase that has a figurative meaning or the associative or connotative meaning.
Example: A drop in the ocean
Meaning: A very small part of something.

Example: A piece of cake
Meaning: Easy, simple to do, no difficulties.

An idiom is a figure of speech that does not have the obvious presented meaning. Idioms differ from culture and country, as the phrase uses commonplace objects, people, or animals. Examples and meanings of idioms in American culture include:

• Raining cats and dogs Raining hard
• Monkey around To fool around
• Playing with fire Taking foolish risks
• Jog my memory Help remember something

Idioms and idiomatic expression are meanings that cannot be understood by the dictionary definition of each of the words in the idiom. The sum of the dictionary meanings of all the words would NOT explain the meaning of the expression.
For example, " easygoing "

An English dictionary would explain that "easy " means simple, not difficult ", and that " going " means traveling from one place to another. Thus you could literally translate easygoing as meaning the path from one place to another is not complicated or physically far, by adding together these dictionary meanings. But this interpretation is NOT CORRECT.

Idiomatically, we mean that such a person's personality is warm, friendly, pleasant, and helpful.

To be NOT EASYGOING is to be rude, argumentative, sullen, sarcastic, negative, antagonistic, easily offended, or imagined to be offended, difficult to have a normal conversation with.

Finally, if we DO NOT MEAN THE IDIOMATIC MEANING, then we use the word " literally " to mean the exact dictionary meaning, not any other meaning intended, as in " the hiking trail up the mountain was literally easygoing ".