Long-tail keywords are highly specific search queries that typically consist of three or more words. While they receive significantly lower search volume compared to broad 'head' keywords, they are much less competitive and have a dramatically higher conversion rate.
For example, ranking for 'shoes' (a broad keyword) is incredibly difficult and the user intent is vague. However, ranking for 'men's waterproof trail running shoes size 11' (a long-tail keyword) is easier, and the user searching that phrase is highly motivated and ready to make a purchase.