A histogram is a graph showing how many pixels in a photo fall at each brightness level, from pure black on the left to pure white on the right.
A histogram is the only trustworthy readout of exposure a camera gives you, because the rear screen misleads. On a bright day outdoors the screen looks dim and tempts you to underexpose; in a dark room it looks bright and tempts you to overexpose. The histogram instead plots how many pixels sit at each brightness level, so a spike jammed hard against the right edge means blown highlights with no recoverable detail, and a spike against the left means crushed shadows.
Reading it lets a photographer catch a clipped sky or a blocked-up shadow while still standing in front of the scene, when a reshoot or an exposure tweak costs nothing. Finding the same problem at the computer, after the light has changed or the subject has left, usually means the shot is lost. Mirrorless cameras can show a live histogram before the shutter fires, which turns it from a check into a guide.
A photographer shoots a snowy landscape and the rear screen shows a clean, bright image. The histogram tells a different story: a tall spike pressed against the right wall, meaning large areas of snow have clipped to pure white with no texture. The camera's meter did this on purpose, trying to render the snow as mid-grey, so the fix is to dial in about +1.3 stops of exposure compensation and watch the spike pull left off the edge, bringing the surface detail back.
The same habit catches the opposite case at a concert, where the meter overexposes a dark stage trying to lift the shadows and blows out the performer's spotlit face while the crowd stays black. Checking the histogram after the first frame of any high-contrast scene takes two seconds and saves the whole set.
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