Tire load range or tire ply rating is a designation branded on tire's sidewall that identifies how much load the tire is designed to carry at its industry specified inflation pressure.
In the past, tire load range was directly related to the number of ply layers used to make up the internal structure of a tire. The number of plies was used to determine the relative strength of the tire - the more plies, the stronger the tire. Today they just indicate the equivalent strength. So even if tire is rated as a 10 or 12 ply, it can actually have only 2 or 3 plies. In fact, most radial passenger tires have only 1 or 2 ply layers, and heave duty light truck tires have 2 or 3 plies.
Tires with higher the load range or ply rating can withstand higher inflation pressures and carry heavier loads.
P-Metric passenger tires mostly have Standard Load range, with SL or nothing branded on tire sidewall. Extra Load passenger tires will have XL branded on tire's sidewall service description. Light Load passenger tires will have LL branded on tire's sidewall service description. Light load tires are designed to carry less weight than standard load tires and have been developed for specific applications - typically when relatively large tire sizes are used as Original Equipment tires on relatively small cars or for Track and Competition DOT tires used for racing applications.
Euro-metric tires will have nothing branded on their sidewalls, while Extra Load Euro-metric tires will be branded with XL, and Reinforced tires will be branded with RF in their service descriptions.
Light truck tire load range is identified by a letter after tire size in service description in ascending alphabetical order - the further down the alphabet, the stronger the tire.
It is important to remember that when changing tire sizes or converting from one type of size to another, make sure to confirm that the load index of the new tire is equal to or greater than the load index of the original tire. Also make sure that new tire's load capacity is sufficient enough to carry the vehicle's gross axle weight.