On some seats it says 'ggf. reserviert'. What does this mean? Can I sit in the seat?
“ggf.” stands for gegebenenfalls or “possibly”. It means the seat has been set aside in the railway booking system for last minute reservations. The official website recommends not sitting there because it may actually be reserved. In practice, you can sit there without trouble as long as you are willing leave the seat if someone does show up with a reservation (this is actually one of the tips to find an empty seat I found on a local newspaper's website). By contrast, there are also usually seats where the display is empty and those are definitely not reserved.