This is the width of a new tyre when mounted on the tyre’s measuring rim, including normal sidewalls but not including protective side ribs, bars, raised letters, or decorations. On tubeless tyres with an aspect ratio of 80 or below (as shown here), section width will usually be displayed in millimetres, such as 315. On tube type tyres, the section width is generally displayed in inches, as in the ‘12’ in 12.00R24.
This describes the shape of the cross-section of the tyre. It is the section height divided by the section width multiplied by 100. Generally, truck and bus tube type tyres have 100 as the aspect ratio and are not separately mentioned; for example, 12.00R24.
The letter ‘R’ indicates radial construction. Bias ply construction is indicated by a dash (-).
This is the distance, in inches, from one bead base to the other (approximately the diameter of the hole that you see in the tyre). In this case, the rim diameter is 22.5 inches.