Short answers, pulled from the story.
The Hundred Days marked the period between Napoleon's return to Paris on the 20th of March 1815 and the second restoration of King Louis XVIII on the 8th of July 1815, a span of 110 days. The name came from a speech by Gaspard, count of Chabrol, prefect of Paris, who used the phrase "one hundred days" when welcoming Louis XVIII back to the capital.
Napoleon left Elba on the 26th of February 1815 after concluding that dissatisfaction in France with the Bourbon restoration, combined with tensions among the Great Powers at the Congress of Vienna, made a popular rising likely. He also calculated that tens of thousands of French prisoners returning from captivity in Russia, Germany, Britain, and Spain would instantly provide him with an experienced army.
Napoleon sailed from Portoferraio with roughly 1,000 men aboard the brig Inconstant and several small vessels, landing at Golfe-Juan on the 1st of March 1815. Royalist troops deployed to stop him switched sides; at Laffrey, he stepped in front of soldiers, tore open his coat, and dared anyone to shoot their emperor. Marshal Ney, who had promised to bring Napoleon to Paris in an iron cage, handed over 6,000 men and joined him on the 14th of March.
The Acte additionel was a supplementary constitution drafted by Benjamin Constant and promulgated by Napoleon during the Hundred Days. It created a hereditary Chamber of Peers and a Chamber of Representatives elected by the empire's electoral colleges. Later historians including Agatha Ramm noted it extended the franchise and explicitly guaranteed press freedom. A national plebiscite returned 1,532,527 votes in favour, less than half the turnout of the earlier Consulat plebiscites.
Napoleon's hemorrhoids had become severe enough by June 1815 to prevent him from sitting on a horse for more than very short periods. Observers including Carnot, Pasquier, and Lavalette believed he had prematurely aged during his exile on Elba, and Sir Neil Campbell had noted he had grown inactive and corpulent there. During Waterloo, his inability to survey his troops from horseback directly interfered with his ability to exercise command.
The Treaty of Paris, signed on the 20th of November 1815, reduced France to its 1790 boundaries, stripping the territorial gains made by the Revolutionary armies in 1790-1792. France was ordered to pay 700 million francs in indemnities across five yearly installments and to maintain a Coalition army of occupation of 150,000 soldiers on its eastern border territories for up to five years, at French expense.