Overexposure in highlights or blacks — irretrievable data loss. Histogram shows it, zebras warn you, but once clipped, it's gone.
Overexposure in the highlights or shadows irrevocably destroys image information. You see it in the histogram as clipped edges—at the top in the highlights, at the bottom in the blacks—and later in the edit, no LUT or color grading can help you anymore. The pixel-level information is gone. That's why classic exposure measurement had to work on set before digital seemed to make everything possible.
In practice, clipping often happens when you let sunlight directly into the lens or have to shoot a scene with extreme dynamic range—a face in shadow, a window behind the person. Your camera only records what the sensor has captured. If the brightness is too high, the photodiodes lock up. If it's too low, you get noise to the point of being unrecognizable. Modern cameras offer zebras—animated stripes over overexposed or underexposed zones—to give you a live warning. But that's just a hint. Real control happens via the histogram: flat and continuous in the middle = good; peaks on the left or right that end abruptly = clipping.
On set, this means: adjust exposure, use ND filters, adapt artificial light, or restage the scene. In the edit, you need headroom—intentionally expose slightly darker to work in post-production. This is also called exposure latitude, and high-quality cameras with large latitude give you more room to maneuver. RAW formats offer more data and thus more chances for recovery than compressed codecs, but even RAW has a physical limit.
Common mistake: Beginners only look at the monitor and think everything is fine. But monitors lie. The histogram is your only honest friend. Those who expose conservatively and systematically avoid clipping have freedom instead of frustration later in color grading—and that is the gift of the smart cinematographer to the colorist.