A long-tail keyword is a keyword phrase that contains at least three words (though some say two or more is considered long-tail).
Long-tail keywords are used to target niche demographics rather than mass audiences. In other words, they're more specific and often less competitive than generic keyword terms. Because of that, they provide both short-term and long-term benefits.
Since they're very specific, they allow you to rank highly in search results for popular topics relatively quickly while still gaining ground on head terms. Plus, they tend to attract highly qualified traffic that's more likely to convert to leads and customers.