[susan53]

A concordancer is simply a software programme which analyses a bank (or corpus) of authentic language data. Feed in any word or phrase and it will pull out all the examples of that word which are contained in the corpus or corpora that you are searching.
That means you see exactly how the word is really used in the language and not just how people think it is used (or even worse, how people think it should be used)- and that's why it's reliable.