When you first plug your iPhone or iPad into a computer, you’ll be prompted to “trust” the computer on your device. This gives iTunes and other management tools on the computer the ability to access your photos, files, settings, contacts, and other data. Your iPhone or iPad remembers this decision and will automatically trust that computer in the future.
If you change your mind later—maybe you trusted iTunes on a friend’s computer and want to revoke that computer’s access, or maybe you accidentally trusted something you shouldn’t have—there is a way to do so. But it isn’t obvious.