The Emergency Powers Act was an amendment to the Galactic Constitution passed at the very start of the Separatist Crisis in 24 BBY to allow Sheev Palpatine to remain Supreme Chancellor long after the expiration of his elected terms. The act was originally intended to remain in practice for as long as the Separatist Crisis carried on.
On the eve of the First Battle of Geonosis in 22 BBY, Junior Representative Jar Jar Binks convinced the Senate to increase the scope of the Chancellor's emergency powers because of the apparent threat of the Confederacy of Independent Systems. With the expansion, Palpatine could bypass the Military Creation Act and form the Grand Army of the Republic. Palpatine remained in power as Chancellor over the Clone Wars through the expanded Emergency Powers Act.
Four additional amendments were made to the Constitution and enshrined in law throughout the war, further expanding Palpatine's emergency powers. In effect, Palpatine became the most powerful chancellor since before the Ruusan Reformations. One of the amendments was the Reflex Amendment, which granted the Chancellor near-absolute power over military affairs, almost completely bypassing the Senate. Unbeknownst to the Senate, Palpatine was in truth a Sith Lord named Darth Sidious, and that he had deliberately engineered the Clone Wars so that he could remain in office and plot the downfall of the Jedi Order. Passing the Emergency Powers Act was, in a way, the one of the many steps towards Sidious' goal to topple the Galactic Republic and replace it with the Galactic Empire.
During the restoration of the Galactic Republic in the form of the New Republic, Chancellor Mon Mothma was granted the same emergency powers once granted to Supreme Chancellor Sheev Palpatine by the Emergency Powers Act. Mothma, however, intended to eliminate those powers because of their destructive effect on democracy.