These marks are placed opposite the tire wear indicators located on the tread. Using the indicators, you can quickly find them to assess the degree and uniformity of rubber abrasion.
The gap from the tread surface to the indicators should be the same. If it is different, then this is a sign of the need for a wheel alignment. When the tread has worn down to the indicator, the tire needs to be changed, since the resource has already been used up.
Some manufacturers make an indicator in the form of numbers. In this case, the tires are replaced when the extruded numbers disappear.