What does a deferred action mean for a DUI in the state of Washington?

No such thing by that particular name in Washington. There are three procedural devices used in Washington courts that seem to fit the general idea you are talking about: (1) a deferred prosecution under RCW chapter 10.05 where a defendant states, under oath that he believes he/she is guilty of DUI, that he/she suffers from alcoholism (supported by an evaluation by a professional) and that if the alcoholism is not dealt with, he/she might commit the crime again. If the defendant successfully completes treatment and stays out of trouble for five years, the case is dismissed; (2) depending upon the jurisdiction, a device either called a "continue without finding" (CWOF), a "pretrial diversion agreement" (PDA) or a "stipulated order of continuance" (SOC), where the case is continued without disposition one or two years where if the defendant completes the treatment and stays out of trouble the case is dismissed or reduced to a lesser charge, and (3) a deferred sentence, where the defendant pleads guilty, and after one to five years, depending upon the agreement, the case would be vacated then dismissed if the defendant completes treatement and stays out of trouble.

There is a deferred prosecution, where you are diagnosed as alcoholic and do two years of treatment, stay out of trouble for five years, and then get the case dismissed.