305 / 50R20 is a tire code which explains the size of a tire. This universal standard is used, so different tire manufacturers can produce tires which will fit same cars and rims.
Here’s an explanation of what these metrics mean in practice:
This number tells you what is the tire width in millimeters. In this case we can see it's 305, or 12.0 in inches.
Aspect ratio. Aspect ratio refers to the height of tire sidewall in relation to tire width. In this example, the aspect ratio of 50 and tire width of 305 end up in a sidewall height of 153mm, or 6.0 inches.
The letter in the middle refers to tire construction. Nowadays around 99% of the tires sold for cars are radials.
Sometimes you also see tires with RF or ZR.
RF indicates the tire is run flat, whereas ZR means the tire has speed rating higher than V. These tires are also radials, as the R indicates.
This number refers to rim diameter. It means this tire can be mounted only to a 20" rims.
What does 120Q mean?
120Q tells you the load index and speed index of the tire.
Load index tells you how much weight the tire can support when fully inflated. It's not meant to be a precise number per se, but more of an indicator on what size vehicles the tire is meant to be used in.
Speed index refers to the speed the tire has been designed to be used in. Usually the speed index is way higher than the speed limits, so in a normal traffic it's usually not needed to worry about having a tire with too small speed index. However, if the tires are used occasionally on track days, having tires with appropriate speed index is usually a smart move.
120Q in numbers
The load index 120 means that the tire has been designed to support 3086 lbs or 1400 kg.
The speed index Q means that the tire has been designed to be used in speed up to 100 mph or 160 kph.