These units allow you to specify sizes in terms of percentages of the viewport width and viewport height. See also the test page without a meta viewport.
The allowed units are:
vw: hundredths of the viewport width.
vh: hundredths of the viewport height.
vmin: hundredths of whichever is smaller, the viewport width or height.
vmax: hundredths of whichever is larger, the viewport width or height.
Here are some examples.
50vw means 50% of the viewport width.
Don’t forget to resize your browser window to see the boxes resize automatically.