The first letter (only found on certain tires) indicates the vehicle type. P stands for passenger vehicles, and LT refers to light trucks.
The first number tells you the width of the tire tread in millimetres. Usually it will be between 155 and 245.
The second number indicates the height of the tire sidewall as a percentage of the tire’s width. Usually it’s between 45 and 80.
The letter R means it’s a radial tire. If not a radial tire, it will be a bias tire. These two tire types are manufactured differently.
The last number provides the inner diameter of the tire, a.k.a. the wheel’s diameter, in inches.