In Shotcut, as well as in video compositing software, a keyframe is a frame used to indicate the beginning or end of a change made to a parameter. For example, a keyframe could be set to indicate the point at which audio will have faded up or down to a certain level.
Or, Keyframing is the simplest form of animating a change to a parameter.
Keyframing in a compositing software:-
Example of Result of keyframing in shotcut:-
Image source from Zoom Image and then Pan - #2 by samth