An idiom is a group of words (a phrase) that can’t be figured out by looking up each individual word. Its meaning is different from the literal meaning. For example, when we say that someone has egg on her face we don’t mean that someone is literally walking around with egg over her face; what we mean is that someone feels embarrassed because she has done something wrong.
Idioms are very easy if you have grown up with them and speak the language, but think of people learning English and hearing things like “you are doing my head in”. they must think we are nuts!