Twotterwrench gave great info. Just adding to that:
1. Fuel is your only energy source.
2. Burning one pound of fuel requires the oxygen from 15 pounds of air.
3. Since only about 30% of the air is used for combustion, then 50 pounds of air has to pass.
4. Until now we have only talked about the engine core. On a modern turbofan engine typically 6 times more air passes the fan only.
5. So one pound of fuel typically "moves" 350 pounds of air. That's a lot of air!!!
6. A mid sized tubofan engine burns several pounds of fuel per second at take-off thrust at sea level.
Moving air is the whole name of the game.
Accellerating a 100 tons heavy airliner from zero to 500 mph means for instance that 100 tons of air has to be accellerated backwards at a speed of 500 mph. This is just a hypotetical example which doesn't account for drag. Lifting the same airliner from the runway to 30,000 feet requires much more.
Now you may think that it is important to accellerate the air to as great speed as possible. That's not the case. It would be terribly noisy. Therefore it is better to accellerate more air to a lower speed.
The very largest engines may "treat" as much as roughly 100,000 ft3 air per second. Quite amazing, even if only some 4 or 5% of that is used for combustion. At lower power settings the percentage of "burned air" is lower.
Regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs