Here is a good commentary on the SYG law from the Volokh Conspiracy blog. This part kind of sums things up nicely.
However, it goes even further and discusses why Zimmerman wasn't arrested which is something else I want to try and explain but could never seem to word it correctly so I would leave it out. That being the need for probable cause to arrest someone.
In sum, Florida’s non-retreat rule is not some 21st century novelty. It is consistent with a long tradition of American law, in which different states have had a variety of rules about when, if ever, retreat might be required.
Even among the most restrictive states, such as New York, retreat in safety is not required before using deadly force in the home; to prevent a burglary (if the person reasonably believes that the criminal would use force to thwart the person’s termination of the burglary) ; to prevent a robbery ; or to prevent a kidnapping, forcible rape, or other forcible criminal sexual attack. Thus, whether you are in Lake Placid, New York, or Lake Placid, Florida, and someone attempts to rob you when you are walking down the street, you have no duty to retreat before using deadly force to thwart the robbery.
However, it goes even further and discusses why Zimmerman wasn't arrested which is something else I want to try and explain but could never seem to word it correctly so I would leave it out. That being the need for probable cause to arrest someone.
(2) A law enforcement agency may use standard procedures for investigating the use of force as described in subsection (1), but the agency may not arrest the person for using force unless it determines that there is probable cause that the force that was used was unlawful.
The Sanford police said this is why they did not arrest Zimmerman: they did not have probable cause to believe that he had broken the law. In fact, the statute does not change the law, but it apparently is effective at reminding law enforcement officers of the standard they are required to obey.