What is the "one final judgment" rule?

rule. "[T]he 'one final judgment' rule, [is] a fundamental principle of appellate practice that prohibits review of intermediate rulings by appeal until final resolution of the case. 'The theory is that piecemeal disposition and multiple appeals in a single action would be oppressive and costly, and that a review of intermediate rulings should await the final disposition of the case.' [Citations.]" (Griset v. Fair Political Practices Com. supra, 25 Cal.4th at p. 697.)