Clipping
Clipping is the point where tones in your photo go so light or so dark that no detail remains: pure white or pure black. In the histogram you see it as a peak against the left or right edge. Once clipped, that detail usually can't be recovered.
Turn on the clipping warning (the triangles above the histogram in Lightroom), and overexposed areas turn red while crushed areas turn blue. A little clipping in a bright light source or deep shadow is fine, it sometimes comes with the territory. Mainly avoid it in important areas like a face or a white dress. Hold Alt or Option while moving the whites and blacks sliders to see clipping live as you drag.
Related terms
Prefer a look in one click?
The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.