Slide Master view is a behind-the-scenes view that lets you see and change placeholders and layouts. It's also the place where you can make across-the-board changes, such as adding a logo to all your slides.
Thumbnails of the layouts appear under the slide master because layouts, along with information such as theme colors, fonts, effects, and background styles, are part of the design shown in the slide master.
In Slide Master view, placeholders can contain only text, even though this text doesn't appear in your presentation and doesn't print. Rather, it is custom prompt text, there to remind you of what to do in the placeholder. For example, when creating a presentation in Normal view, you may see Click to add text, which is the default custom prompt text for the text placeholder.
When you finish making your changes to the slide master and the layouts, you return to Normal view, where you can apply any layout, including custom layouts, to any slide.
Now let's see how to create your custom layout.