Trusted devices (you can have up to two) are devices that are tied to your MogoAccount because you've successfully logged in to the mobile app with them. Trusted devices help make sure your account is secure and also make it more convenient for you to log in.
When you're loading the app from a trusted device, there's no need to type in your username (email address) because you're the only one who can log in to a MogoAccount with that device. You simply need to enter your password.
You can remove a trusted device from your Account Settings through the app or on the web at any time. For example, if you lose your mobile phone, log in through the web (mogo.ca) and remove your phone from your trusted devices so that if someone tries to access your MogoAccount through the app, they would have to enter both your username (email address) and your password.
This means that your friend can't log in to their MogoAccount using the app on your phone—unless they've deleted their trusted device first. It also means that once someone logs in to their account on your device, your device will become one of their trusted devices. Confusing? Yeah. So it's probably best not to share.