A histogram shows how many pixels in a picture are at each of its possible brightness levels. Its horizontal axis represents the scale of possible brightness values, usually either for the picture overall or for a single color channel. Its vertical axis represents the pixel frequency in the picture for each of these values.
If the histogram shows too many pixels on the left (that is, in the dark tones) and not enough on the right, then the picture is probably underexposed. If most values are on the right, the picture is probably overexposed. In most cases, a normal photo should make use of the whole range of the histogram.
Histograms are only truly useful for pictures with 24-bit, 32-bit, or 48-bit color, or for grayscale pictures. (However, any picture in a modern photo format will use one of these color types anyway.)
Right-clicking on a histogram will show a context menu where you can switch among several display modes: Brightness, brightness plus channels, channels, red channel, green channel, or blue channel. You can also set the histogram to half size or full size, and choose between a light or dark histogram background.