The purpose of an engine is to push the plane forward. 

Engines function by sucking in air from the front with a fan. The main shaft is spun by an electric motor until the engines are ignited. The air pressure is raised by the compressor. The compressed air then travels through the combustor, where it is mixed with fuel and ignited. The hot air is combined with cold air that bypassed the engine core and produces an exhaust. The air blasted from the nozzle rotates the turbines, which causes the fan and compressor to spin.

Thrust is a forward force produced by the engines to overcome the drag of the aircraft. Due to Newton’s Third Law, when the engines pushes backwards, the air pushes the aircraft forward with an equal magnitude of force. This allows the aircraft to move forward.

Drag is an aerodynamic force exerted by a fluid (liquid or gas) on a solid object. Drag opposes the movement of the aircraft through the air.