Hello gzerovti,
If you use the search function, top right of your screen, you will be able to find a lot of info on this matter. However if you look at the picture below you will see, around the control, three coloured circles. The blue one controls steering, the brown one, astern speed & the green one, ahead speed. In the picture you can see that the starboard engine is running at 100% (full speed) ahead (180 degrees) If you click on the border line between green & brown you will stop the engine. moving into the green area increases ahead speed and into the brown area astern speed. Perhaps you will understand now. But do use the search function to find what you need,that is why it is there.
Angus.