A more thorough explanation:

An accessory-after-the-fact is a person who helps someone who has committed a crime, knowing that they committed the crime, and with the intention of helping them avoid getting caught or punished. This person can be charged with obstruction of justice.

For example, if someone helps their friend hide evidence after they committed a robbery, they could be considered an accessory after the fact. Another example would be if someone gives their family member a place to stay after they committed a murder, knowing that the police are looking for them.

The government has to prove that the person helped the criminal after the crime was committed. The Supreme Court has ruled that an accessory-after-the-fact cannot be held liable as a principal offender, unlike an accessory before the fact.