Momentum and inertia are related. Inertia is the tendency for a body to remain at rest, until and unless a force makes it begin moving. That same tendency works when it is in motion.
The motion of an object will continue until something makes it change its motion. A railroad car, once it gets going, will continue its motion for a long time, until the tiny forces of friction cause it to slow down and stop. This can take miles. Even putting on the brakes can take up to mile, because there is so much momentum.
p = m * v
A force in the same direction as the body is moving, will increase its speed. A force in the opposite direction will slow it down.
A force coming from the side will turn it.