Add a stroke to the bottom edge of an element that is darker than the colour of the element, assuming a top-down light source.
This technique will make the element look like it's popping out of the page. It's often used for navigation bars at the top of the page, to add some depth and separate the navigation from the content without being too obvious/visually heavy.
If the stroke is not the same colour as the element background, but darker, it probably won't look like a darker lower edge, and you'll just be adding a line divider.
