Economies of scale refers to the benefit that larger production volumes allow fixed costs to be spread over more units lowering the average unit costs and offering a competitive price and margin advantage.
Producing in large volume often generates economies of scale. The per-unit cost of something goes down with volume because vendors charge less per unit for larger orders, and often production techniques and facilities cost less per unit as volume increases. Fixed costs are spread over larger volume.
For more on scaling, check out these scaling myths (and how to overcome them) on the LivePlan blog.