Guided Access is used as a temporary "lock" on certain apps. Not only is it used on iPads, it is also available for use on iPhones and iPod touches. If an app is being used under guided access, there is limited use on what can be used on the app and also a time limit can be set. Through this feature, you are able to set a pass code so when the time does expire the students' (or whoever is using the app) is unable to get back into it the app unless they have the pass code.