This graph (click to magnify) shows the 100 most popular searches for “squid” and its variants using Wordtracker’s Keyword Search Tool. The Y-axis is the estimated number of searches per day, and the X-axis shows different searches. The most common searches (“squid” and “giant squid”) are over on the left, and that’s what most squid-related webpages are optimized for. But the “long tail” of slightly less popular searches (trailing off to the right) actually outnumbers the searches for the most popular one or two search phrases.
“Chasing the long tail” means writing your page in a way so that it tends to get all the search traffic for a slightly less popular but still popular search phrase which few other webpages are targeting.