For example lets say I wanted to make snapchat better by adding some extra features to it, so I would create an app that has some cool features that makes your snapchat experience better and it would run the already existing snapchat app in this app that I create which then you could use the extra features that my app has. Also if it had in app purchases or the app would cost a fee how far could this go without breaking the law. Sorry if this is confusing, hopefully I got the main point across.