How it Works
Typically when you install an app from the Play Store you are provided a list of permissions that the app wants access to. In order to install the app, you must by default, accept these requests or no app for you. With Android M this changes, instead when you install the app you don't have to accept the permissions, instead you grant or deny them when the app actually needs it. For example if you download Facebook Messenger and you go to send a picture, the app will pop up a little window asking if you want to give the app permission to view your files. Same goes for using the camera in Messenger, if you want to take a picture it'll ask you for permission to use the camera first. The ability for users to fine tune their app permissions will go a long way for developers and trust.