Short answers, pulled from the story.
Dignitas was a Latin concept referring to a man's personal reputation, moral standing, ethical worth, and the esteem owed to him by virtue of his family's history. It encompassed dignity, prestige, charisma, and power derived from personal respect, and the Oxford Latin Dictionary lists over a dozen approximate translations including rank, status, honour, and worthiness. The word had no direct English equivalent.
Cicero was the most prolific writer on dignitas among all ancient Roman authors. Julius Caesar, Tacitus, and Livy also used the concept extensively in their writings and oratories. Cicero initially linked dignitas to the related term auctoritas, meaning authority.
Dignitas represented a Roman man's good name in its fullest sense: his past and present reputation, his achievements, and the honour of his family. Losing it was understood as a form of social destruction, and historical figures went into exile, committed suicide, or entered armed conflict rather than allow their dignitas to be stripped away. Mark Antony is named in the sources as one who chose suicide to preserve it.
Florus wrote that the stubbornness of Cato the Younger pushed Pompeius Magnus to actively build up his own dignitas through military preparation. Cicero recorded that Caesar could not tolerate anyone claiming an equal share of dignitas with him. Aulus Hirtius described how Marcus Claudius Marcellus built his reputation by turning public opinion against Caesar.
Dignitas never held all of its meanings simultaneously; the term shifted as Roman society, politics, and its key authors evolved. In 46 BC, Cicero noted the concept's ambiguous nature in writing. After Caesar's death, Augustus rejected the contemporary meaning of dignitas and adopted the related term auctoritas instead.
When Cicero's political career ended, he felt unworthy of dignitas in its conventional active sense because he had withdrawn from public life. He redefined dignitas to mean lifetime impact and applied it to his past political influence, while calling his present state of withdrawal otium. This pairing gave dignitas a retrospective quality, allowing it to describe what a man had accomplished rather than only what he was still doing.