In a natural language, an idiom is a phrase that conveys more information than the individual words combined. In order to understand an idiom, you must have knowledge of the culture of the person using the idiom, as well as a good grasp of the language.
Programming idioms are similar. They involve uses of particular aspects of the language in ways that convey extra information to the reader of the code. Sometimes idioms are used to expand the abilities of a language. Other times, idioms are used to restrict what you can do with a construct. In all cases, however, the idiom relies on an understanding of the culture from which it is formed.