What does it mean that the vice president is “only a heartbeat away” from the presidency?

It means that the vice president is really close to the presidency. meaning if anything happens to the president, the vice president, becomes president. they literally mean "a heartbeat away"

The haitian revolution was the result of a long struggle on the part of the slaves in the french colony of st. domingue, but was also propelled by the free mulattoes who had long faced the trials of being denoted as semi-citizens. this revolt was not unique, as there were several rebellions of its kind against the institution of plantation slavery in the caribbean, but the haitian revolution the most successful. this had a great deal to do with the influence of the french revolution, as it to inspire events in haiti. the haitian revolution would go on to serve as a model for those affected by slavery throughout the world.