The first law of states that an object in motion, will remain in motion until acted upon.
Gravity is always acting on an object. Here gravity is the only force acting on a till rocket keeping it down on the ground.
But in Space there is no gravity around there is no force stopping the rocket. See, when a rockets launches there is gravity and thrust both acting on it. but since there is no gravity in space the rocket will thrust forward and continue traveling forever unless an outside object collides with it.
If the rocket were flying on Earth thrusters are needed to have the rocket accelerate since gravity is always acting on it
Newton's Second Law* Law of Acceleration *
Less force/thrust needed
The second law states that the acceleration of an object, increases with increased force.
More force/thrust needed
This means that the heavier the rocket, or the more stuff a rocket carries, the more force or thrust means the rocket gains more acceleration.
The third law of motion states that for every action, there is an equal and opposite reaction.
... so more thrust and force is needed to counteract the gravity to have the rocket gain acceleration
Gravity is always acting on the rocket while on Earth...
In order for the rocket to launch the gas is being pushed against the thruster causing the thruster to push the exhaust gas out. This causes the rocket to move in the opposite direction of the exhaust gas.