In space there is no atmospheric pressure that can push you or which mass you can use to repel on. All propellant (reaction mass) is from on-board. The less on-board mass, the better. To get a most efficient space flight: accelerate a small amount of propellant mass to extremely high exhaust velocity.

This drawing of a rocket combustion chamber & nozzle, is while in the atmosphere. The gas pressure inside will decrease as energy is used to accelerate the gas. The area of the nozzle where the gas pressure is equal to atmospheric pressure will be the nozzle exit area.

The function of the nozzle is to convert the chemical-thermal energy generated in the combustion chamber into kinetic energy. The nozzle converts the high pressure & high temperature gas in the combustion chamber into high velocity gas of lower pressure and temperature. The drop in temperature of the combustion gases flowing through the nozzle is high and can be as much as 2000-3000 F. Since the gases in the combustion chamber nay be at 4000-6000 F, the gas temperature at the nozzle exit is still about 3000 F.
Since thrust is the product of mass (the amount of gas flowing through the nozzle) and velocity, a very high gas velocity is desirable. Gas velocities from one to two miles per second (5000 to 12000 feet per second) can be obtained in rocket nozzles.

Nozzles which perform this seemingly amazing feat are called DeLaval nozzles.

The limits of chemical rocket engines.
With increasing speed it becomes harder and harder to gain another mile per hour. This is because the amount of fuel one has to carry becomes really big, and it becomes difficult and expensive to lift that much fuel into space. Solar escape velocity is nearing the practical limit of how fast one can move with conventional rockets.