Senators were individuals who represented the many planets of the the galaxy in the Senate of the Galactic Republic. Many senators supported and cheered Palpatine's proclamation that turned the Republic into the Galactic Empire at the end of the Clone Wars. The senators then became members of the Imperial Senate.
Fourteen years into Imperial Senate's existence, the power of the senators had grown weaker as the Emperor's planetary governors and Moffs exercised more responsibility over their planets and systems. A few years later, the Imperial Senate was disbanded by the Emperor.
Shortly after the Battle of Endor, the Rebel Alliance achieved their goal and restored the Republic in the form of the New Republic. As one of its first acts in the long process of bringing back democracy to the galaxy, the Republic restored the Galactic Senate.