A rocket sitting on the pad has two main forces on it: thrust T pushing it up, and weight W = m g pulling it down (g = 9.81 m/s² on Earth). Newton’s second law says the net force makes it accelerate:

a = (T − W) / m = T/m − g

If T < W, the rocket stays on the pad, however strong the engine sounds. If T = W, the acceleration is zero: the rocket could hover, if the thrust stayed exactly equal to the weight. Only when T is bigger than W does it rise.