20 is the height of the tire in inches.
11 is the width of the tire in inches.
9 is the diameter of rim the tire fits on in inches.
This tire would typically be mounted on a 9x8 or 9x9 wheel. The tire width is typically 2-4 inches wider than the wheel width. So a 10", 11", or 12" wide tire could be mounted on a 8" or 9" wide wheel. The last number in the tire size and the first number of the wheel size must always match.