Pointer Cancellation requires that clicking or tapping only completes an action when the pointer is released (the up-event), not the instant it makes contact (the down-event). This gives users a chance to slide their finger or cursor off the target before letting go, canceling an accidental activation.
This matters most for people with tremors or limited fine motor control, who may touch the wrong target before correcting their aim. Interfaces that fire on touchstart or mousedown, common in some custom-built buttons and drag interactions, fail this criterion.