The Master Slides is the design template or design theme used for the slides within your presentation. There are four different master slides — title master, notes master, handout master and the most common , the slide master.
The default design template when you first start a PowerPoint presentation is a plain, white slide. This plain, white slide and the font choices used on it were created in the slide master. All Slides in a presentation are created using the fonts, colors and graphics in the slide master, with the exception of the Title slide (Which uses the title master). Each new slide that you create takes on these aspects.