A long-tail keyword is a very targeted search phrase that contains three or more words. It often contains a head term, which is a more generic search term, plus one or two additional words that refine the search term. For example:
Head term: unicorn
Long-tail keywords: unicorn games online, unicorn costumes for kids, unicorn videos on YouTube
Long-tail keywords are more specific, which means visitors that land on your website from a long-tail search term are more qualified, and consequently, more likely to convert.