Picture a graph: the y-axis is popularity (i.e. numbers sold) and the x-axis is products (e.g. each point is a book). If all the most popular products (e.g. Harry Potter) are closer to the y-axis, and as you move away from the y-axis popularity decreases, you get a long tail on the graph.
The idea here is that stocking, e.g., a few copies of a LOT of relatively unpopular books, allows you to cater to niche markets and can significantly increase profits compared to only carrying products that are in high demand.
Companies like Amazon are masters at exploiting the long tail. Oh, and
here is the original article describing this idea.