hi! Yes, this concept is very important for a web site development. It concerns a strategy of development in terms of content using the keywords used by your visitors to find your website. The main idea is to use the keywords combinations that send to your site not so much visits to develop your content and thus your traffic. Watch some videos on long tail keyword research with different points of view (the second video is great).
Further to the longtail notion, I think its worth saying that it involves targetting less popular keywords / phrases i.e. not the top 20 / 30 as their cumulative sum in terms of total search volume will actually be greater than the top key phrases. For example it has been said that Amazon actually makes 57% of sales from keywords that are not the so called popular keywords. Its a good way to start an SEO campaign for very competitive markets, in my opinion.
Danyo is right, but let's try an example. Obviously, you have a bodybuilding site. You could try creating one page and optimize it for the keyword "bodybuilding", but you would probably end up in the SERP's at number 999,999 out of 1 million pages. How much traffic do you think you will get? Not much. Or you could create several pages, each optimized to a different long tail keyword, like "bodybuilding for arms", "bodybuilding for legs", or, "bodybuilding for back muscles", etc. By the way, don't use these, I just made them up. This gives you 2 advantages. The SE's list "pages", not "sites". While each of these pages won't get much traffic, they will be listed higher because there is less competition. When totaled up, the traffic to all of your pages should be significant. The second advantage is that your site will have more content, therefore more pages. This in itself will improve the ranking of all of your pages. I agree that it's a good way to start an SEO campaign in a competitive market.
Basically, long tail keywords are, like the above posters mentioned, less popular keywords or under targeted keywords. These are keywords that are phrases and usually have 3 or 4 words. Since you have a site on body building you could use the Google Keyword tool to find searches related to body building and find the keywords that have less amount of searches (they are usually the ones that have less competition too). To find competition type the keyword phrase into Google with " " and see how many competing pages there are. If under 200K I would write a blog post or article with that keyword phrase in the title. If you do this with many under targeted long tail keywords you will find that your blog post or article will be on page one of Google. Which means more traffic for you.