What does the liter's stand for in vehicles?

It is the measure of the volume capacity of the cylinders in the engine. A 1.5 Liter engine means that, in total, the volume of the cylinders sum up to 1.5 Liters. 1 Liter is equivalent to 1000 cubic centimeter (cc), and you may notice that some vehicles (especially motorbikes) are rated by cc. A 1000 cc motorbike means that it has a 1 Liter engine. I hope that helped