The surface of the pad pushes the rocket up while gravity tries to pull it down. As the engines are ignited, the thrust from the rocket unbalances the forces, and the rocket travels upward. Later, when the rocket runs out of fuel, it slows down, stops at the highest point of flight, then falls back to the Earth. Force in the equation can be thought of as the thrust of the rocket engine. Mass in the equation is the amount of rocket fuel being burned and converted into gas that expands and then escapes from the rocket.
Acceleration is the rate at which the gas escapes.
Inside the rocket, the gas does not really move, but as it leaves the engine it picks up speed. For every action there is always an opposite and equal reaction. With rockets, the action is the expelling of gas out of the engine. The reaction is the movement of the rocket in the opposite direction. To enable a rocket to lift off from the launch pad, the action, or thrust, from the engine must be greater than the weight of the rocket.