Camera screens lie. In bright sunlight they look dim; at night they look brilliant; and their brightness slider has fooled countless photographers into underexposing an entire vacation. The histogram — that small mountain-range graph your camera can overlay — never lies, and five minutes of understanding it will save you years of ruined highlights.
What you are looking at: the graph maps every pixel’s brightness from pure black on the left edge to pure white on the right. The peaks show where most of your image’s tones live. There is no single “correct” shape — a beach scene piles up bright pixels on the right, a candlelit dinner leans left. Shape follows scene.
The edges are what matter. A pile of pixels climbing the far right wall means clipped highlights: detail that has become featureless white and can never be recovered, not by any software, ever. Blown skies and glowing white shirts live here. A pile against the left wall means crushed blacks — less catastrophic, since shadows hide their sins, but still lost information.
The working rule: expose to protect the right. Digital sensors, like slide film, forgive underexposure far more than overexposure. When a scene’s contrast exceeds what the sensor can hold, sacrifice the shadows and save the highlights — a slightly dark file lifts cleanly in editing; a clipped bridal gown is gone forever.
Turn on the blinkies. Most cameras offer highlight alerts — clipped areas flash in playback. Between the histogram and the blinkies, you have a complete honesty toolkit that works in any lighting the screen cannot handle.
The habit to build: after any shot that matters, glance at the graph rather than the picture. Pretty-on-screen is an opinion; the histogram is a measurement. Trust the measurement, and reshoot while the moment is still in front of you rather than mourning it in the edit.