What does "gules" mean?

Gules: The tincture red, indicated in seals and engraved figures of escutcheons by parallel vertical lines. Hence, used poetically for a red color or that which is red. His sev'n-fold targe a field of gules did stain In which two swords he bore; his word, "Divide and reign." P. Fletcher. Follow thy drum; With man's blood paint the ground; gules, gules. Shak. Let's march to rest and set in gules, like suns. Beau. & Fl.