Every design template in PowerPoint includes a slide master. The master stores the styles for the design template. It's the place you go in any presentation to change aspects of the design that's applied.
The picture shows the relationship between the slide master and a slide in the show. The fonts and font styles that are in place on the master for the title and body text are reflected in the title text and bulleted points on the slide.
So if you want to change the font for the title text in the show, you'd just change the title text's font on the master. You can make other presentation-wide changes here, too, such as applying a background design or arranging and sizing placeholders.
While you can change the master to effect design changes throughout any presentation, saving those changes as a template preserves them as a design that can be applied to any presentation.
The slide master sits behind the scenes, in its own place, called master view (more on that in the next section). It is never part of the slide show that people see.