Guided Access, once turned on, keeps users from errantly clicking or tapping within an app and ending up someplace they're not supposed to be, or deleting something accidentally.
First, you need to go to Settings > General > Accessibility > Guided Access and turn it on. You'll also want to turn on the Accessibility Shortcut.
Triple-click the home button in whichever app you want to turn it on for. From here, you can select any buttons that you don't want clicked. Or you can also disable the home button while keeping the touchscreen on.