In scanner histogram/levels controls, the x-axis is tone value or brightness (often luminance, or per-color-channel brightness if you’re viewing channels separately). The y-axis is how many pixels fall at each brightness.

The left slider sets the black point: that input value becomes pure black, and anything darker is clipped. The right slider sets the white point: that input value becomes pure white, and anything brighter is clipped. The middle slider adjusts midtones/gamma.

For many normal scans, moving black and white points in to the edges of the histogram is a reasonable starting point because it uses the available tonal range well. But it is not always correct. If the original scene is intentionally low contrast, stretching it to full black and white can make it look wrong.

Defaults may be inside the histogram because some images benefit from a little clipping to increase contrast, especially B&W scans. In color images, aggressive clipping can lose highlight/shadow detail and can shift color if channels clip unevenly.

Workflow matters too: if you’re making a finished JPEG, set the points for the look you want. If you’re scanning at high bit depth for later editing, it’s often better to avoid heavy clipping and preserve as much data as possible.