This image of the Archangel Michael shows him in his role as heavenly warrior. Although to our modern-day eyes, he may not look like a mighty fighter, medieval images of the archangel emphasized a sense of his spiritual power, rather than any physical force. Michael is depicted in this sculpture standing on top of a small beast, which was meant to represent a dragon - another description of Satan.
In this way, Michael represents the fight of good against evil, and is shown victorious over his enemy. St. Michael would have held a lance in his upraised right hand, with which he would have speared the dragon beneath him. The format of showing the victor standing on top of his defeated enemy is a very old one, and medieval artists also used scale to indicate that the power of God (as represented by the champion, Archangel Michael) is stronger than the power of Satan - who is a very small dragon crushed underfoot.