Trebuchets were built in front of the castles that were under seige - they were too bulky to cart about.
A framework was put up to take a pivoted arm. A weighted box was attached to the short end. A net was looped over the other end containing a missile.
This might be a stone or lump of metal, or burning wood to try and set fire to thatched rooves in the castle. Sometimes decomposing, dead animals were lobbed in to spread disease!
The missile end of the arm was roped, and wound down, taut to the ground.
It was fired by cutting the taut rope. The weighted box suddenly dropped, and the long arm flew into the air. The looped net flung open and the missile was hurled towards the castle.