What are idioms?

Idioms are usually defined as groups of words whose meaning is different from the individual
words.

So, under the weather has nothing to do with the literal meaning of 'the weather'; it means 'feeling ill'. If you sweep something under the carpet, you try to keep something secret; and if you put someone in the picture, you give them the information they need to understand a situation.

As these examples illustrate, in some idioms the meaning can be almost impossible to guess out of context, while others are more transparent.