It simply means that no Marine ever lets another Marine down. Its meaning has been well-earned. This applies to all activities at all times. It must never be relaxed, particularly during times of hardship, discomfort, or danger. Justice, consistency, firmness, and respect are the roots of discipline.
Men like to serve in a well-disciplined unit.
Mob methods disgust them. Exercise and display absolute loyalty toward a superior, particularly when he is absent. This is not only morally correct, it is the only sure footing in any military organization. It also enhances your personal prestige among your subordinates. Always show enthusiasm - it is infectious.