SLA means service level agreement. when you set up SLA you will get a notification every time a call is in the queue longer than the specified SLA.
SLA is used to make sure that your callers are not in the queue longer than the time you have specified.
For example you declare that all calls within your organisation are answered within 3 minutes. So you set the SLA in the queue for 180 seconds. Once that time is breached the queue manager will receive a notification notifying that a call has breached the SLA.