Concept stuff and ideas yes! If it's a story-based game (like a visual novel or RPG) and you're writing the whole script beforehand then no (but you can plan the story!). Basically any final art or dialog or code specific to the game should only be created during the jam period.
The theme is "unexpected" - it's known in advance (so you can brainstorm) and I trust that no one will start on their game until the actual jam period :)
