You create a menue in your web page, that is not made of simple text links, but instead there is a small image for each link that changes slightly wenn hovering over it with the mouse.
Each image is placed via the "background"-attribute of the "div"-tag. When hovering over that image, the "hover"-method calls the 2nd image, that shall be shown.
Now there is the problem, that when loading the page for the very first time, only the 1st image of every link is loaded. The 2nd ones only gets loaded when hovering over them the first time. And exactly in that moment a very very short delay in loading the 2nd image ist noticeable.