In order for an airplane to fly at all, air must flow over and under its wings. The more aerodynamic, or streamlined, the airplane is, the less resistance it has against the air. If air can move around the airplane easier, the airplane's engines have less work to do. This means the engines do not have to be as big or eat up as much fuel which makes the airplane more lightweight and easier to fly. Engineers have to think about what type of airplane they are designing because certain airplanes need to be aerodynamic in certain ways.
Propulsion is the study of what kind of engine and power an airplane needs.
An airplane needs to have the right kind of engine for the kind of job that it has. A passenger jet carries many passengers and a lot of heavy cargo over long distances so its engines need to use fuel very efficently. Military airplanes flying secret missions do not want to give away their location in the sky so their engines minimize their exhaust to prevent detection. Engineers are also trying to make airplane engines quieter so they do not bother the passengers onboard or the neighborhoods they are flying over. Another important concern is making the exhaust cleaner and more environmently friendly.