Master slides define background and common objects such as logos, headers, and footers for your slides. They provide a uniform appearance for your entire project.

All the artefacts on a master slide, such as, objects and placeholders reflect on the slide that is linked with the master slide.

Every Captivate theme comes with a set of master slides that you can use and customize.

When you work on your project slides, use the Property Inspector (Window > Properties) to:

• Link to a different master slide: In the Master Slide drop-down list, make appropriate selection. You can also apply a Master Slide to a group of slides.
• Display master slide objects on top of the objects in your slide: Check Master Slide Object on Top.

Note: Objects on the main master slide are always below the objects on other master slides in the Z-order.

Master slide maintains its own timeline. Therefore, master slide objects are not shown on the timeline of the linked slide. You cannot adjust the timeline of master slide objects or change their Z-order on a project slide timeline.

Use master slide background: Select this option to use background color or background image from master slide.

For more information on using master slides, see https://helpx.adobe.com/captivate/using/master-slides.html