A slide master is the top slide in a hierarchy of slides that stores information about the theme and slide layouts of a presentation, including the background, color, fonts, effects, placeholder sizes, and positioning. The key benefit to using a slide master is one can make universal changes to every slide in the presentation without having to manually change each individual slide. Any new slide added to the presentation will have the same theme as the slide master. If you create a slide master after you build individual slides, some of the items on the slides may not conform to the slide master design.

You are able to change the background while you have a certain theme on you like for all the slides and the theme and new background will both be on the slides.

To make changes to your slides, in the Slides area, select the desired slide master layout
HINT: You can work on the slide master just as you would a regular slide