A histogram is a graph of brightness values: dark tones are on the left, bright tones on the right.

A quick rule of thumb:

• Mostly pushed left: the image may be underexposed
• Mostly pushed right: the image may be overexposed

But this is only a guide, not a rule. A dark scene can have a left-heavy histogram and still be correctly exposed, and a bright scene can be right-heavy and still be correct.

The more important warning sign is clipping:

• If the graph is piled up or cut off at the left edge, shadow detail may be lost to pure black.
• If it is piled up or cut off at the right edge, highlight detail may be lost to pure white.

That happens because cameras can only record a limited brightness range. When the scene exceeds that range, detail is lost at one or both ends.

So: don’t judge exposure by histogram position alone. Use it mainly to check whether important highlights or shadows are clipped, unless that pure black or pure white look is intentional.