In other words, it adds content to a page that is otherwise hidden; it requres a mouseover to show the text.
Once the mouseOver event happened on a graphic, whatever image was there before was replaced by a different image file.
The setrollover function is where you tell the computer what image to "switch" to on the mouseOver event.
The word usage examples above have been gathered from various sources to reflect current and historial usage. They do not represent the opinions of YourDictionary.com.