An accessory to a crime refers to a person who helps with the carrying out of a crime. As opposed to the person who actually commits the crime, an accessory to a crime is a person who makes committing the crime easier for the person who is actually committing the crime.
In general, a person who is convicted for being an accessory to a crime will not be punished quite as severely as the person who actually commits the crime. However, this is not always the case. Depending on the crime, an accessory to a crime can be found to be just as culpable for a crime as the person who commits the crime. For example, an accessory to a crime could be found just as guilty as the person who committed the crime is the accessory to the crime helps by doing something very grave such as hiding a dead body or providing a murder weapon while under the impression that the weapon will be used to kill another person.