The question should be what is the difference between being an accomplice to robbery and an accessory after the fact to robbery. An accomplice is someone who assists in a crime by somehow aiding or assisting (aiding and abetting) the "perpetrator" in its commission. Someone who helps the perpetrator commit the crime is equally liable for it, and may be punished just the same as the perpetrator. Penal Code section 211 defines robbery. Section 212.5 lists the degrees, and section 213 spells out the punishment, which is different for first and second degree robbery. An accessory after the fact is someone who helps the principles (perpetrator(s) and accomplice(s)) after the crime is complete, for example, by hiding someone who participated in the crime, knowing they participated, so they will not be found. Generally, the crime of being an accessory after the fact is found at Penal Code section 32. Section 32 is a wobbler, meaning it may be charged as a misdemeanor (up to a year in jail) or a felony (16 mos, 2 or 3 yrs).
This is not intended as a legal opinion nor does the answer to this question create an attorney-client relationship, or give rise to a privilege. It is not a confidential communication.
Accessory after the fact means that the crime has already been committed and is completed. Then steps are taken by someone who did not commit the robbery to help cover it up. For example one might conceal or destroy evidence that the police are looking for that could link the crime to the perpetrators.