It's pretty simple really, onMouseOver and onMouseOut are events that are triggered when the obvious happens: the mouse enters or leaves the image boundary. document is the current HTML file, image is the name of the image we're working with, and src is the value of the image. Assigning to document.image.src changes the image, and we only assign to it during an event. Notice in the second line that the name of the image is set and the original src is off.gif.