Note: This functionality will only be available for you if you've also added the Planning Booster to your licence.

In the calendar, you'll see a 'Planning' bar below each day of the week. This bar shows your total availability for that day, based on the amount of hours per week you have set. The bar will fill up based on the tasks, meetings and calls you have planned in your calendar for that day. If you’ve selected multiple calendars at once then everyone's total availability will be added up.
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When the bar is green, it means everything's okay and you have some room left in your schedule to plan more work. Red means you're already overbooked for that day in particular. If you hover over the bar, you'll see the amount of hours/minutes you're overbooked.

The availability bars also take all planned tasks, meetings and calls from your planning into account. So even if a task is not exactly scheduled in your calendar, but it is due on that day and planned via the planning, your availability decreases.