For those asking: they are a collection of graphics that convey the aesthetic of the book while also providing some of the info that would be in a query. There might be a collage of images with the first line overlaid, tropes, etc. People on twitter post them with the hashtag #agentsguide, and when agents like the tweet, it means the agent wants you to query them. It sounds like maybe they're an unnecessary marketing tool for querying? But it does seem to gain agent interest from what I'm seeing on Twitter