Accountability when there is a personal data breach

A personal data breach is a breach of security leading to the accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or access to personal data transmitted, stored or otherwise processed. Note, it is not to do with purely the loss of personal data, or unauthorised disclosure of personal data, but also unlawful destruction and alteration.
What does it mean that we have 72 hours to report a breach?