The rev counter indicates the number of engine revolutions per minute.
The start of the red zone on the dial indicates the maximum engine speed which may be used briefly in all gears when the engine is warm and after it has been run in properly. However, it is advisable to change up a gear or move the selector lever to D/S (or lift your foot off the accelerator) before the needle reaches the red zone.
CAUTION
The rev counter needle must only ever briefly go into the red zone on the scale; otherwise there is a risk of engine damage. The start of the red zone on the dial is different for some engine versions.
For the sake of the environment
Changing up a gear early will help you to save fuel and minimise engine noise.