A cover is like an introduction to your page. A cover will disappear underneath your content when you scroll down.

For example you can have a full-bleed image (row background image) and some text on top as a cover for a project.

Activate the Cover feature in "Lay Options" → "Activate Cover Feature"

The first row of your layout will be your cover

When the Fullscreen Slider Addon is active the cover will behave like a default row for that page.