This determines how long a booking lasts. The duration can be admin defined (fixed block) or customer defined (they need to input on the front-end). Duration units can be Hours, Minutes, Days or Months.
Fixed blocks have a set duration that you define. The customer can only choose one block which will last the defined amount of time. For example hair dresser appointment lasts one hour and you can only book one at a time.
Customer-defined blocks have a set duration, but the customer can choose how many blocks they want. For example, the coworking space is rented out in hourly intervals, and the customer can define how many 1 hour blocks they would like to rent.
If you set your Booking duration to be customer defined, you can include a minimum and maximum allowed value in the General tab. This allows for a more flexible starting schedule. You can define 30-minute blocks and set the minimum to 2 blocks. Your customers now need to book at least one hour, and can add increments of 30 minutes.