You adjust the image tonal scale using the tone controls in the Basic tab.
When you click Auto at the top of the tone controls section of the Basic tab, Camera Raw analyzes the camera raw image and makes automatic adjustments to the tone controls (Exposure, Recovery, Fill Light, Blacks, Brightness, and Contrast).
When you adjust tone automatically, Camera Raw ignores any adjustments previously made in other tabs (such as fine-tuning of tone in the Curves tab). For this reason, you should usually apply automatic tone adjustments first—if at all—to get an initial approximation of the best settings for your image. If you are very careful during shooting and have deliberately shot with different exposures, you probably don’t want to undo that work by applying automatic tone adjustments. On the other hand, you can always try clicking Auto and then undo the adjustments if you don’t like them.
As you make adjustments, keep an eye on the end points of the histogram, or use the shadow and highlight clipping previews.