Click the View tab to see checkboxes that will allow you to turn on the Ruler, Grid, and Guides (click the image below to see a screenshot).
Ruler: Allows you to see the dimensions of your slide. You'll see a vertical and horizontal ruler.
Grid: By default, the gridlines are 1 inch apart. Right click in white space of your poster to get more options for spacing. This enables precise alignment.
Guides: By default, you'll get one horizontal and one vertical guide placed in the center of your poster. Right click on a guide to add more guidelines, or to delete one. You can use Guides to invisibly define columns of your poster, margins, and more. This gives you manual control, alternatively, you can use Smart Guides (see below).
Smart Guides: Powerpoint has a built-in system for showing you alignment as you move objects around. The video below demonstrates what Smart Guides look like.