A long tail keyword is a keyword consisting of two or more words. Often these words have less competition which makes it easier to score well for them. This is exactly why it is wise to focus on long tail keywords.
As an example:
Long tail:
Red nike shoe without laces
Short tail:
Shoe
The long tail variant displays a much more specific view. We can therefore assume here that the searcher is out to buy the red shoe. With the short tail variant, it is not yet entirely clear to the buyer exactly what he or she wants.
By using long tail keywords, you can receive more purchase-oriented visitors and therefore get more out of your content strategy.