How does propulsion work?
Specific impulse is the change in momentum per unit mass for rocket fuels, or rather how much more push accumulates as you use that fuel.
The speed of a rocket depends on thrust (which is roughly the amount of propellant that is thrown out of the back of the rocket and the speed at which that propellant is thrown out) compared to the rocket's weight.
The faster the speed at which propellant is thrown out the back of the rocket, the faster the rocket can travel or the more cargo it can carry. The specific impulse of a rocket propellant is a rough measure of how fast the propellant is ejected out of the back of the rocket. A rocket with a high specific impulse doesn't need as much fuel as a rocket with low specific impulse. The higher the specific impulse, the more push you get for the fuel that rushes out. Or, put another way, specific impulse determines how much fuel you have to use to get a good-sized push.
What is a propellant?
How are rockets designed?
What are the types of rocket propulsion?
Why is mass important?
What is thrust?
How do you calculate specific impulse?
How do you calculate rocket engine performance?