Wrappers are used to enclose large areas of content. There are a couple of versions, o-wrapper--main for the main content on the page, and o-wrapper--wide for a wrapper that stretches the full width of the page instead of being constrained to 1200px as the normal wrapper is.
Wrappers are often given the class .o-grid in order to be the container for .o-grid__rows.
The div with the class .o-wrapper--main should also have the class .js-wrapper--main in order for it to pick up on the page height fix.
<div class="o-wrapper">...</div>
<div class="o-wrapper o-wrapper--wide">...</div>
<div class="o-wrapper o-wrapper--main js-wrapper--main">...</div>