At midnight every night during specified time periods, you’ll receive a bonus task without having to spin a PokéStop. These tasks are different from Field Research tasks acquired from PokéStops and involve activities that you can complete from wherever you are. 

If you don’t have an open Field Research slot, then a fourth slot will open up to accommodate the bonus task for you. If you already have four slots filled up, you’ll need to complete one Field Research task (either a bonus task or one acquired from a PokéStop) to free up space for another bonus task.