Long tail keywords are multi-word keyword phrases, typically four or more words, and very specific.
(Example: “inhaled saline for cystic fibrosis in babies”.)
Or they may be short keyword phrases that are very specific or contain a product name.
(Example: “blue iPod Nano”.)
Each long tail keyphrase tends to receive only a few searches per month. But it can be easy for a publisher to make the top 10 in Google for these phrases because not many other web pages are likely to contain them.
So although each long tail phrase brings you only a small amount of traffic per month, the cumulative effect of having thousands of long tail keywords on your site is having thousands of targeted visitors - without even trying.
Long tail keywords tend to emerge naturally as you write about various topics in your niche.
You probably don't need to make a huge effort to create content that focuses on them.
However, it doesn't hurt to comb through your keyword research and pick out some targeted longtail-type keywords that people may search for in your niche, and toss them into your content. Particularly think about long tail keywords that solve problems in your niche, that your audience may be searching for.
Learn more about the long tail and what it means to online businesses in this Wikipedia article: