Car tyres need to be able to withstand different-sized loads. This means there needs to be different types of tyres, as the tyres used for a large vehicle, an SUV or a fully loaded transporter have to take more weight than those of a normal car. XL tyres have been especially developed for use with heavy loads.
Generally speaking, the tyre name XL is used for heavy duty tyres. However, the labelling varies between tyre manufacturers. Sometimes you might have XL tyres that are also labelled "reinforced" (RF or RFD), or models labelled EL (Extra Load).