I wish I could be brilliant on this but the answer is simple. It is when a governmental or a quasi governmental entity and take action but defers it to a later time. It can be as simple as lack of resources to commence action or as complicated as witness protection and the US Marshals. It can involve a limited immunity or deferment on a traffic ticket. Prosecution of one offense with probation is the most common deferment for the second offense.
In immigration it is a favor shown to the individual. There is no right to it. It is never required. But why deport a good fellow? And why so hurt a child? But if they screw up just one tiny bit -- bye bye.