Problem of the day is a special problem designed to help you practice what you've already learned. You can access if from your Study plan.
It changes every day (you can see a timer that tells you when a new problem will be available). Problem of the day is picked automatically from the pool of unsolved problems from the topics you previously completed.
Keep in mind, if you haven't completed enough topics you might not have any Problems of the day or have the same one on repeat.