thrust
Thrust is the forward push a rocket or jet engine produces. It is the force that lifts a rocket off the pad and shoves an airliner down the runway — the muscle of any reaction engine. The faster and the more heavily an engine throws mass out the back, the more thrust it makes.
Thrust is the rate at which the engine hands out backward momentum, and by the impulse-momentum theorem that rate is a force. For an engine ejecting propellant, thrust = v_ex times (dm/dt): the exhaust speed v_ex multiplied by the mass flow rate dm/dt (kilograms of exhaust per second). Its units are newtons (N), like any force. By Newton's third law it is simply the reaction to the engine pushing the exhaust backward — the exhaust pushes the engine forward just as hard.
For a rocket to lift off, its thrust must exceed its weight; the leftover force is what accelerates it upward. Engineers boost thrust by making the exhaust faster or the mass flow bigger — which is exactly why rocket nozzles are shaped to speed the gas up as much as possible before it leaves.
An engine burns 250 kg of propellant per second and expels it at 2800 m/s. Thrust = 2800 times 250 = 700000 N. If the loaded rocket weighs 500000 N (mass about 51000 kg), the net upward force is 200000 N, giving an extra acceleration of a = 200000 / 51000 = about 3.9 m/s^2 on top of overcoming gravity.
Thrust is mass flow times exhaust speed; only the part above the weight accelerates the rocket.
Thrust is a force (measured in newtons), not a speed and not an energy. And it must beat the rocket's weight to get it off the ground — an engine with lots of thrust but less than the vehicle's weight will only sit there roaring.