Spoiler warning: Plot and/or ending details follow.
The Red Lanterns or Red Lantern Corps were, in an alternate reality, a group of powerful beings who utilized a facet of the energy wielded by the ancient Oan civilization, who were proclaimed as the Guardians of the Universe.
In variations of this particular reality, the Red Lanterns drew energy and used anger and rage to focus this energy through individual power rings. The red light energy required the user to utilize such emotional states to focus this spectrum's facet of primordial source energy from the origin of that multiverse, through Red Lantern power rings. (TOS - The Spectrum War comics: "Issue 1", "Issue 2")