In the Presell System, a presell report is a lead magnet, usually in PDF format, as little as 10 or as long as 30 pages long, designed to build your authority and pre-sell readers in a systematic way.
A good presell report gradually moves readers closer to the point where they're ready to buy. It establishes all the points your prospects need to buy into, before they can accept your product as the one for them.
In particular, the report should revolve around a Bridging Idea... and when people buy into this idea, buying what you have to offer becomes the next natural and logical step.
Finding the ideal Bridging Idea that presells your product can be tricky, but is worthwhile, because it means people who read your report are much more likely to be pre-sold on your product... and since most of your competitors probably aren't doing this right now, you'll have a competitive advantage. (Details of how to construct your Bridging Idea can be found in Core Component 5 of the Presell System.)