Spot color printing is a printing process in which each color is printed using only its own ink. That might seem logical, but think about how a computer prints color documents. Digital printing is kind of like mixing red and yellow watercolor paints in kindergarten. For example, orange is a mix of magenta and yellow inks. With spot color printing, on the other hand, that orange ink comes straight from a bottle.
Spot color printing is most often used on promotional items that have a one-color design in one location. More than three colors, and spot color printing gets pretty pricey.