those markings indicate the maximum vehicle speeds for the corresponding gears. At these speeds in these gears, the engine will be at redline. These are used as guides when upshifting manually, if you don't have a tachometer.

but even if you have a tachometer, it is a useful guide when downshifting. For example, if you are in 4th and the speedo needle is below the III marking, then it is safe to downshift to 3rd. Without these markings, it is difficult to calculate if you will overrev the engine in the lower gear at the same vehicle speed.