Reaching space is hard. Lots of energy needs to be released to do so. Since the start of the space era, rocket engines have been producing this energy by chemical reaction known as combustion. Combustion requires two chemical reactants, an oxidizer and a fuel, called propellants. The heat carried by the combustion byproducts is turned into kinetic energy by letting the hot and pressurized gas to expand through the nozzle.
To put payload into orbit, one must use a large quantity of propellants. And every bit of mass must be useful to obtain an efficient rocket. This constraint led to design rockets with two or more stages so that the structural mass that became useless because the tanks have been emptied, can be jettisoned.
Currently, space transportation industry is dominated by two propulsion technologies: the liquid propellant engine and the solid propellant engine. Both of these technologies have their own advantages but they come with tradeoffs and drawbacks. They are far from ideal propulsion systems.
A third propulsion technology exists, called hybrid. Hybrid Rocket Engines have the potential of featuring the advantages of both liquid and solid propulsion technologies. They could become the best propulsion technology for space transportation in the near future!
Adapted from : Fundamentals of Hybrid Rocket Combustion and Propulsion - Chiaverini, M. I. and Kuo, K. K.