Long tail strategy in marketing refers to focusing on products and services that are demanded by a niche market. It is a marketing strategy using which you can either exceed the demand for popular goods or complement the sale of popular goods.
Though the popular products make most revenue, the not so popular products can generate equal or more revenue when combined together.
https://www.feedough.com/what-is-long-tail-in-marketing-long-tail-theory-explained/ will help you more on the topic.