What are long-tail keywords?
Keywords can be broad and far-reaching (these are usually called "head keywords"), or they can be a more specific combination of several terms — these are often called "long-tail keywords."
(https://moz-static.s3.amazonaws.com/learn/seo/Keyword-fundamental-pages/Longtail-Graph.png?mtime=20170413074235)
Singular keywords might appear to be your ultimate goal as they often have temptingly high search volume. However, they usually have extremely tough competition. You may want your boutique clothing store to rank for "clothes," but it's going to be tough to rank above Zappos and Nordstrom.
On top of that strong competition, singular keywords can be infuriatingly vague. If someone is searching for "dog," you don't know if they want a list of dog breeds, information about dog food, a place to buy a dog collar, or just a site with cute photos of dogs.
Long-tail keywords usually have more clearly defined intent. For example, "best organic dog food for a puppy," or "inexpensive dog walkers Seattle." You'll also find that long-tail keywords have less competition, with room for a smaller site to break in and make their mark on the SERPs.
Source:MOZ