"stuff you didn't want" would be accidentally recorded background noise, or code that makes the payload compatible with a device other than the one you're using

"stuff you didn't want" would be accidentally recorded background noise, or code that makes the payload compatible with a device other than the one you're using. This is data specifically crafted to get your computer to do something in addition to what you asked it to do, without your consent, and not in a way that benefits you The word for data like that is "malware".