A 'Cut Crease' is usually for a dramatic effect to define eyeshadow contrasts for a shadow-like but defined look. It essentially contrast the eyeshadow colours in the 'crease' of the eyes to define the whole makeup look.
If the 'cut crease' technique is not perfected, it can end up ruining the whole look or appear funny and unclean making the otherwise smooth transition of hues impossible (which ends up tacky).
In an easy tutorial Jackie Aina shows how to cut crease in a neutral hue for a glam finish. Watch and learn how in super easy steps above.