You can use histograms to judge exposure.
Here's the histogram for the normal exposure for the castor bean leaves.
There are shadows (left side) and midtones (middle), but no highlights (right side).
The next histogram shows pronounced overexposure of the leaves.
Note the tall spike at the right side of the histogram.
The spike is called clipping.
Clipping means that available image data has not being recorded.
The next histogram shows severe underexposure.
Note the tall spike, now at the left side of the histogram.
Again, the spike represents clipping.
Image data has been lost.
You can also use histograms to evaluate the contrast of a scene.
The photograph of the CD has lots of bright and dark areas.
There's lots of contrast.
On the histogram, the contrast is seen as the tall spikes in the shadows (left side) and the bright midtones (to the right).