Normally, the rewards inside are based on the master loot table for *all* chests - if the item can appear in any random chest in the universe you can also get it from a Reward Bag. Most of the time, your reward will be a Tech Card, Manipulator Module, Upgrade Module, or Teleporter Core, or a random vanity item.
On very rare occasions, much like chests across the universe, you can obtain something much more rare and impressive.
Additionally, if you obtain a Reward Bag from a themed NPC, the loot table for their specific themed chests is also added to the possible drops and you are more likely to get one of these rarer rewards. For example, completing quests from Glitch Castles on Volcanic worlds can reward you with procedurally generated Tier 6 weapons.