This happens if you have added less estimated hours than expected for the task. By default, the system takes working hours as 8 hours per day.
For instance:
Suppose you have assigned a task to a user for 10 days. So the estimated hours should be 80 hours (10*8=80). But if you have entered 40 hours as the estimated hours, then the resource will be booked for 5 days and rest 5 days will be marked as Available. Please refer below image (Refer Image 1) for more details-