A shadow is a soft dark shape behind an element that makes it look lifted from the page. It is like the shadow under a real box resting on a table.
Designers may use a shadow to make a menu or dialog stand out from the content behind it. A subtle shadow can suggest which layer is in front without drawing a visible border.
Use shadows sparingly and keep them consistent. If every item looks raised, nothing feels special, and strong shadows can make a page look busy or harder to read.