SCP-3007 is basically a hallucination suffered by randomly-selected people during which they will see themselves in a ruined alien city and if they're hurt in the hallucination then they are hurt in real life too. (I could be wrong on this part but) allegedly anyone that knows about the hallucinations has a chance of experiencing them themselves, meaning that if hallucinating people start telling the world about what they're seeing, soon everyone will know about the hallucinations and everyone will start to hallucinate, at which point The Red will be able to take over the planet.

Either way, the

TL;DR: Scary red energy monster that appears to people in visions and tries to spread to more people until it takes over the planet.