It was actually about all kinds of information about the safety of their cars, how the company handles collisions, how they decide to make human drivers take over, details of injuries caused by their cars, etc.

The DMV required them to submit their safety data before allowing the vehicles on the road. Waymo claims that the data the DMV needed about the public safety of their cars, and the emails they exchanged about it with regulators, were entirely "trade secrets" to keep them hidden from the public who understandably felt like they should be able to access that information since those cars are going to be on their streets.