A more thorough explanation:

An accessory before-the-fact is a person who helps or encourages someone else to commit a crime, but is not present when the crime is committed. This person can be held responsible for the crime just like the person who actually committed it.

John wants to rob a bank, but he needs someone to help him plan the robbery. He talks to his friend Mike, who agrees to help him come up with a plan. Mike is an accessory before-the-fact because he helped John plan the crime, even though he wasn't there when it happened.

Another example could be a person who gives someone else a weapon to commit a crime, or helps them get away after the crime has been committed.

These examples show how someone can be held responsible for a crime even if they didn't actually commit it themselves. It's important to remember that being an accessory before-the-fact is a serious crime and can result in punishment just like the person who committed the crime.