Overrides allow you to define settings which are overriden on all the slides in your presentation. For example, you can increase the font size on all slides.
The following overrides exist:
This is a percentage value to change the font size on all slides
Override Slide Animations
This allows you to set the default transition for your slides if none is set on the slide itself.
Override Element Animations
This allows you to set the default element animation if none is set on the element itself
Hide on all Slides
This will hide the author and the slide number on all slides