A more thorough explanation:

An accessory is a person who helps or contributes to the commission or concealment of a crime. There are two types of accessories: accessory before-the-fact and accessory after-the-fact. Unlike an accomplice, an accessory does not need to be present during the crime.

An accessory before-the-fact is someone who encourages, aids, or assists in any material way in the commission of a crime, thereby "participating in the design of the crime." The aid or counsel may be far removed in time from the commission of the crime, although it must be shown to have retained some relationship to it by causing, encouraging, or assisting the offense.

For example, if someone helps plan a bank robbery by providing information about the bank's security system, they could be considered an accessory before-the-fact.

An accessory after-the-fact is someone who, knowing a crime was committed, receives, relieves, comforts, or assists the offender or in any manner aids them to escape arrest or punishment. The aid provided by the defendant to the principal must be given after the principal completes the crime.

For example, if someone helps a friend who just committed a robbery by hiding them from the police, they could be considered an accessory after-the-fact.

To convict someone of being an accessory after-the-fact, the government must prove that the defendant had knowledge of the original crime and acted with this knowledge when assisting the principal.

Overall, an accessory is someone who helps or contributes to a crime, either before or after it is committed.