The clipped shadow and highlight indicators in the Editor allow you to easily see which areas of the image are too dark or too bright. Highlighted areas are shaded according to the much they transgress the thresholds.
The thresholds for these indicators are defined in Preferences > General.
The clipped shadow indicator will highlight areas where all three channels fall at or below the specified shadow threshold.
The clipped highlight indicator will highlight areas where at least one channel lies at or above the specified highlight threshold. If you want to see only where all channels are clipped, then enable the luminosity preview mode in addition to the clipped highlight indicator.
Clipping is calculated using data which depends on the state of the gamut button which you can toggle above the main preview in the Editor tab. When the gamut button is enabled the working profile is used, otherwise the gamma-corrected output profile is used.