The int's for QualitySettings.SetQualityLevel(int)

I wonder what the numbers (int’s) are for

QualitySettings.SetQualityLevel();

As I want to change the graphics in an options menu. I’d really like to know this.

It depends on how many quality levels you have. If you have five, for example, then you could use any int from 0 to 4, which corresponds to each quality level.