The rev counter indicates the number of engine revolutions per minute.
You should select a lower gear if the engine speed drops below 1500 rpm. The start of the red zone on the dial indicates the maximum engine speed which may be used briefly 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” (or lift your foot off the accelerator) before the needle reaches the red zone.
Never allow the rev counter needle to go into the red zone on the dial for more than a very brief period: there is a risk of damaging the engine. 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.