The tyre size on the sidewall is to inform its owner about its parameters.
European tyre markings meet the ERE-R 30 standards. These standards specify markings at the tyre sidewall, where are the most important informations, like tyre size
195 - tyre’s width. The key value in the size of a tyre is the first one and measured in millimetres. The width is changing per 10 millimetres for every size up. This is why we have models like: 175/65 R14, 205/55 R16 or 195/50 R15. The real value is different from nominal and differs according to the manufacturer. 195 says that the width between outer and inner longest points on the sidewall is 195 millimetres when fitted to a proper wheel.
50 - tyre profile. The second value is the height of a sidewall. It is measured by a percentage amount taken from the tyre’s width. Let’s use this example: 195/50 R15 means that the 50% from 195 is 97,50 millimetres. Other profiles such as: 50, 45, 40 are characteristic to low-profile tyres. Models considered as full-profiled are often without profile size and it looks like this: 165R13, as the tyre has a profile of 80.
R - radial tyre. Tyre's radial construction means that the pile cords ar at 90 degrees to the bead. In the tyre description the radial build of a tyre is indicated by the R. In the tyre195/50 R15 carcas is radiating from the centre of the wheel. 98% of tyres sold are radial tyres.
The number 15 - indicates the size of the wheel on which the tyre is to be placed. Manufacturers give the size in inches.