A moral imperative is a strongly-felt principle that compels a person "in question" to act.
Kant took the imperative to be a dictate of pure reason, in its practical aspect.
Later thinkers took the imperative to originate in conscience, as the divine voice speaking through the human spirit.
An example of following a moral imperative is breaking into someone's house in order to save a baby in a burning crib.
An example of not following a moral imperative is making a promise, that you do not intend to keep, in order to get something.