Image -> Adjust -> Brightness/Contrast...
This dialog allows you to change contrast and brightness of the image. This is a very simple and fast adjusting of the tonal range of the whole image.
As you move the sliders you immediately see the result on the original image. Also the Tone Curve shows you the Input/Output relations. On the X axis is the values of original image, the Y axis shows the adjusted values of the result image.
Contrast is the difference between light and dark tones. By moving the Contrast slider up you can see that the ton curve is changing the angle and becoming more vertical. That means more values around middle (midtones) from original image are mapped into larger tonal scale (stretched around the gray middle) on output.
Brightness moves the tonal curve up and down shifting the input image more to bright or dark side.