Short answers, pulled from the story.
The Acte de déchéance de l'Empereur was a law passed by the Sénat conservateur on the 2nd of April 1814 that formally deposed Napoleon I as Emperor of the French. It abolished the succession rights of the House of Bonaparte and released the French people and army from their oath of loyalty to him.
Talleyrand and Barthélemy, president of the Senate, jointly proposed the motion on the morning of the 2nd of April 1814. Talleyrand had already been elected chief of a five-member provisional government by the Senate the previous day.
The Senate charged Napoleon with numerous violations of the Constitution of the Year XII, the same constitution the Senate had itself approved and upheld during his reign. The full text of those reasons was voted on the 3rd of April and published in Le Moniteur Universel on the 4th of April 1814.
Napoleon attempted to place his son on the throne before abdicating unconditionally on the 11th of April 1814. The Allies allowed him to keep his imperial title but exiled him to Elba. He returned on the 26th of February 1815, was defeated at the Battle of Waterloo, and was subsequently imprisoned on Saint Helena for life.
The act called for the Count of Provence to take the throne as King Louis XVIII, representing a restoration of the Bourbon dynasty. Talleyrand had argued for this outcome directly to Frederick William III of Prussia and Alexander I of Russia at a dinner on the 31st of March 1814.
In the years following Napoleon's removal, the act served as a litmus test of allegiance to the House of Bourbon. A person's attitude toward the act was read as a direct indicator of where their political loyalties lay.