The term long-tail-keyword comes from the field of search engine optimisation. If you have a website yourself, then you are probably well aware of the struggle for placement on various terms on google. A long-tail-keyword is a combination of several search words. This is to increase the findability in the search engines even in the case of hard competition for one of the more frequent keywords. Example? Imagine that you wanted to get on the first page of Google with the term "insurance". We would actually make a bet with you that this would be virtually impossible for normal mortals. But: An unusual combination such as "where can I find the cheapest insurance" is more likely to be a bit of a rip-off. The general traffic is certainly lower, but the chance of a good ranking is definitely higher.