"Literal. To make a long story short, the God of Control was able to merge Mementos with the real world for a while, and due to people no longer believing we existed, we were erased from cognition. Thankfully we all ended up in the Velvet Room, and after Yaldabaoth tried to make a deal with me, I managed to encourage everyone to get back up and fight. We managed to get back and wake everyone up, as well as fight and defeat him."

It's very much the cliff notes version. Ren's not sure how much he knows about the future from Crow, as well as from being here for years.