There are several different methods and apps for keeping some content on your phone safer than the rest, and on the Galaxy S6 Samsung has a built-in option called Private Mode. With a tap of a quick setting toggle and authentication you can unlock data from a variety of Samsung's apps that you've previously marked as private, keeping it away from anyone who happens to have your phone. It isn't comprehensive and won't fit everyone's needs, but it's worth knowing about for occasional use.

Understanding and enabling Private Mode