What is the definition of "a person aids and abets the commission of a crime"?

"[A] person aids and abets the commission of a crime when he or she, acting with (1) knowledge of the unlawful purpose of the perpetrator; and [with] (2) the intent or purpose of committing, encouraging, or facilitating the commission of the offense, (3) by act or advice aids, promotes, encourages or instigates, the commission of the crime." (People v. Beeman (1984) 35 Cal.3d 547, 561.)