A rather prestigious mathematical journal asked me recently to referee a paper. More precisely, they asked me to provide "a brief (but relatively quick) report".
Does this mean that this is only a pre-referee report? What should be included in such a report?
A very likely possibility is that they have already contacted two referees, but have received no answers from one of them, or conflicting answers. So what they want from you is a quick general opinion on the paper to help them make a decision; someone has already looked at the technical parts in more detail, so you should focus on evaluating the level of the paper. Since the manuscript has already been sitting on their desk for a while, they want it fast.
Another possibility is that the paper has just been submitted, the editor looked at it quickly and it seemed a quick reject, but he/she wants a confirmation. So in this sense it may be a "pre-report"; if you don't answer negatively, you may be asked to write a more detailed report. But the other possibility seems more likely to me.