Viewport units in CSS, such as 'vw' (viewport width) and 'vh' (viewport height), are relative to the size of the user's viewport. One unit on the viewport width (1vw) is equal to 1% of the viewport's width. Similarly, one unit on the viewport height (1vh) is equal to 1% of the viewport's height. These units are incredibly useful for creating responsive designs that adapt to the user's screen size.