A quad rocket is like a little toy spaceship that people make to go up really high in the sky. It has four separate rocket engines, which are like little boosters that make it go fast, and they all work together to make the quad rocket go up into space.
The first engine makes it lift off the ground and go up a little bit, then the other engines turn on one by one to help it go higher and higher. When it gets high enough, the quad rocket can go into space and float around like a little astronaut.
However, building a quad rocket is very tricky and needs a lot of careful planning and preparation. People have to make sure all the engines and parts are working perfectly together so that it doesn't explode or crash back down to Earth. Also, quad rockets are very expensive to build, which is why only a few people can afford to build them.