Placeholders are outlined with a dashed border. To resize a placeholder, click its border, and then point to a sizing handle on a corner or side. When the pointer becomes a two-headed arrow, drag the handle. To move a placeholder, click its border, and when the pointer becomes a four-headed arrow, drag the placeholder to a new position.
Layout refers to how the various placeholders are arranged on the slide.
For example, in the Default Design template, the title text is in a placeholder that runs across the top of the slide. The body text placeholder encompasses the bulk of the slide; it's the area for all the levels of bulleted points. Each footer is in a placeholder.
Click Play to see an animation that shows you what the placeholders on the slide are, and how to resize and move them, effectively changing the slide layout.