So, the galaxy is divided in four layers more or less based on the distance from the galactic core. The further you get from the core, the more advanced your species is and can be. For reference Terra is in the Slow Zone, between the Unthinking Depths and the Beyond, far from the unimaginable Trancendence. Some human civilizations have made it to the different levels of the Beyond, which is also divided into low, middle and top levels with variance in the technology levels. One of them has managed to travel to the highest zone and pokes with things they don't understand, releasing some Ancient Evil.

The fun trick is that the higher-level tech doesn't work in the lower level zones, so the superfancy FTL and natural AI breaks down or just simply stops working in the lower zone, leaving the advanced folks stranded. The species living in the higher levels also don't really care about the lower zones that much, as they've got their appendages full of everyday work in their own level, maybe slowly working towards leveling up.

One of the things I greatly appreciate in Vinge's writing is the lack of exposition. Yes, that makes getting the universe a bit slower, but I enjoy that much more than infodumps. I also have read Vigne's A Deepness in the Sky before this, so a couple of mysteries were spoiled already. This didn't negatively affect my reading, anyway, so I imagine the reading order is unimportant.