I think there's a world where we'll look back on our advertising-saturated era with the same bewilderment with which we now regard cigarette smoke, child labor, or public executions: a barbaric practice that we allowed to continue far too long because we couldn't imagine an alternative.
Sideloading is simply the act of installing apps on your computing device that haven't been explicitly reviewed and approved by the company who made the device (or the operating system running on it).