What is the "good faith exception" to the exclusionary rule?

The rule announced in United States v. Leon, supra, and Massachusetts v. Sheppard, supra, has come to be known as the "good faith exception" to the exclusionary rule. In People v. MacAvoy, supra, 162 Cal.App.3d at page 764, 209 Cal.Rptr. 34 the court defined "the essential prerequisite to the reasonable good faith exception" as a finding that "the officers had a good faith objectively reasonable belief that the search they conducted was authorized by a valid warrant [and] ... 'that the officers properly executed the warrant and searched only those places and for those objects that it was reasonable to believe were covered by the warrant.' [Citation.]"