Crushing is what happens when you have the ‘brightness’ control of your TV turned down too low, or the ‘contrast’ control turned up too high.
Ignore the names. The brightness control does control the brightness … of the black levels. The contrast control can control the contrast, but what it really does is control the white levels. If you turn the brightness control down too much, then the near-black bits of the picture will become fully black, and therefore merge into it, becoming indistinguishable. So details in darker scenes may become quite impossible to see.
This is called ‘crushing’. Near blacks get ‘crushed’ into fully black.
The same can happen at the white end. Turn up the ‘contrast’ control too high, and near white becomes indistinguishable from full white. Consider this scene from The Lord of the Rings: The Two Towers:
It’s all too easy to simply make Gandalf’s eyes disappear by having the ‘contrast’ too high, ‘crushing’ the whites.