It will start off slow, it then accelerates thus is goes faster. It will eventually reach a point that it is travelling at a constant velocity. Also referred to as Terminal Velocity.
The rocket starts slow because it is still speeding up, from stationary, it starts to move. The engines will propel the rocket and it will speed it up. The constant velocity (terminal velocity) is achieved because the forces at work on the rocket is balanced. The force from the engine is equal to the opposing force which (in the earth's atmosphere) is air (air resistance).