For example, if you put a Microsoft Word document in the Start Up folder, Word will run and automatically open that document at bootup; if you put a WAV file there, your audio software will play the music at bootup, and if you put a Web-page Favourites there, Internet Explorer (or your own choice of a browser) will run and open that Web page for you when the computer starts up. (The examples cited here could just as easily be shortcuts to a WAV file or a Word document, and so on.)
Places that viruses and trojans hide on start up is a guide to places that malware can hide.