A CSS overlay is an empty HTML element that is styled with CSS to be overlaid on top of some sort of background image or video. They typically have some sort of dark background associated with them, and are then made transparent using the CSS property
Opacity determines how opaque or how transparent an element is. It accepts values 0-1, 0 being completely transparent, and 1 being completely opaque. This video will show you how to create your own overlay element with position absolute and opacity properties so you can get a good contrast going between your text elements and the background video behind them.