"Surfaces at different elevations do the following:

• Allow surfaces to move in front of and behind other surfaces, such as content scrolling behind app bars
• Reflect spatial relationships, such as how a floating action button’s shadow indicates it is separate from a card collection
• Focus attention on the highest elevation, such as a dialog temporarily appearing in front of other surfaces

Elevation can be depicted using shadows or other visual cues, such as surface fills or opacities."

Material Design shows elevation using shadows.