Unassigned and Available shifts are similar in a sense that both are not assigned to an employee. The difference is when you might want to use one or the other.
Unassigned shifts are only visible to managers; they serve as a reminder of which shifts need to be covered that day or week, and need to be assigned or made available to be visible on employees' schedules.
Available shifts, while also often unassigned, are visible to employees and are available for grabs. They serve as a tool to fill shifts that are currently open. Employees can apply to those shifts, helping you to get your schedule fully staffed.
Note that both unassigned and available shifts will be labeled as having "no employee" in the month view of the schedule. Switch to the week view in order to clearly distinguish which shifts are unassigned, available, or assigned.