That is a measurement in millimeters...
The numbers on a tire are: the width in millimeters/ height measured as a percentage of the width/ the rim diameter.
So say a 305/65/R15 has a width of 305 millimeters, a height that is 65% of that (198.25 millimeters) and a 15 inch diameter rim.
Edit: 305 mm = about 12 inches(12.00785)
mm x .03937 = inches