The atmosphere contains enough particles which slow the spacecraft preventing it from orbiting the planet. A propulsion device must strain against gravity to rise above the atmosphere. Second, the satellite must also be provided with enough horizontal velocity above the atmosphere to at least equal the local circular speed upon orbital injection otherwise it will reenter the atmosphere and burn due to friction. Both of these jobs are done by rockets. A simple rocket is usually a tall cylinder containing propellant.
Fuel is the item which b urns to provide rocket thrust.
In a simple liquid rocket it is stored in its own separate fuel supply tank. To support fuel combustion the rocket also contains a source of oxygen needed after the spacecraft passes above the atmosphere and cannot collect oxygen in any form. The nozzle is tapered in the middle in an area called the throat to allow the exhaust gas to build up even more pressure and to increase its flow rate out into the wider portion of the nozzle. The gas goes into the wide nozzle portion very fast and produces a force called thrust. If the thrust is greater than the rocket's weight, the craft will lift off.