The words in them do not mean what they ought to mean.
An idiom cannot be understood literally.
The meaning of the whole, is not the sum of the meaning of the parts, but is something apparently quite unconnected to them.
Idioms are mostly phrases that can have a literal meaning in one context but a totally different sense in another.
It does not mean what you would expect it to mean.
Although it is in the form of a phrase, it has many of the characteristics of a single word.
In short, idioms are just like our Malay simpulan bahasa.