A long tail query is a search query that is not very common or 'hot' and most of the time sites do not optimise for these long tails. Long tail queries are very important though, they will give your site a stable rate of traffic. The problems with the hot search terms is that you get a lot of competition and your site might drop in the serp for this one term for a multitude of reasons. Long tail search queries on the other hand brings traffic because a user was searching for a term like "wast management in Europe Germany bananas". Wastmanagagement itself might or might not be a competitive term. But the long tail is certainly not and guess what not many people search for this. However if your site is about wast management and contains information about Germany and just happes to also sell bananans you might just get a hit for this term.
There is actually only one thing you can do to optimise for the long tail search query, write good content! Yes I have said it before 'The number one seo tip write good content' is also good for long tail search queries. The more content you have the more long tails you get and the more stable your search engine traffic becomes. Do not just go for the big search terms, go after the long tails.