The Daily Planner in Harry Potter: Hogwarts Mystery is a new series of three activities that reset each day. They will require you to do things like gain stars in classes, feed your Magical Creatures, and collect Secret Energy.
Upon completion each activity will grant small rewards like 75 coins or 4 energy. Completing all three grants an additional bonus reward such as gems or notebooks. In the picture above one blue notebook is rewarded for completing all three activities.
The rewards from the Daily Planner might not seem like much, but consider that they’re just a free reward for doing activities you would be doing each day regardless.