A colour printed with its own pre-mixed ink instead of being built from CMYK dots.
Spot inks can reach colours outside the process gamut (fluorescents, metallics, particular brand hues) and print as solid areas without a dot pattern. Each adds a printing plate and cost.
A spot-colour reference number specifies an ink, not a screen colour; its on-screen preview is an approximation.
A brand orange that CMYK cannot reach is printed as a spot colour on packaging.
It is the usual answer when a colour must be consistent across print runs or cannot be reached in process colour.