When you create a task, the task estimate will show as this red number. For example, a two-hour task will have two hours left to plan. When you plan this task for two hours, the task becomes fully planned, and the red number will change into a green badge. But when you plan a three-hour task into a two-hour study block, you'll have one hour left to plan to make sure the task is fully planned. Drag the remaining hour into another study block.