A guardian angel is an angel (a created, non-human, non-corporeal being) that has been assigned to guard a particular person, especially with respect to helping that person avoid spiritual dangers and achieve salvation. The angel may also help the person avoid physical dangers, particularly if this will help the person achieve salvation.
Your guardian angel never interferes with your free will. As in all things, God preserves your free will and allows you to either cooperate with or reject the graces you receive through your guardian angel. However, your guardian angel can spark your imagination or memory to guide you toward holiness. In particular, guardian angels help us in times of temptation to turn away from evil, and in times of spiritual opportunity to grow in holiness. Guardian angels even aid the souls of their charges when they are in purgatory.