If you have a tire that is, say, 205/45/17.
The 205 represents the width of the tire in millimeters.
The 45 represents the height of the sidewall relative to the tire width, from the rim to the edge of the tread. It's a percentage, so 45 would be 45% of 205mm, which would mean the sidewall is 92mm high (0.45 x 205mm = 92mm).
The 17 is the wheel diameter in inches.
