What is Solid rocket booster?

A solid rocket booster is a large solid propellant motor used to provide thrust in spacecraft launches from initial launch through the first ascent. Many launch vehicles, including the Ariane 5, Atlas V, and space shuttle, have used SRBs to give launch vehicles much of the thrust required to place the vehicle into orbit. The space shuttle used two space shuttle SRBs, which were the largest solid propellant motors ever built and the first designed for recovery and reuse. The propellant for each solid rocket motor on the space shuttle weighed approximately 500,000 kilograms.