Inertia, forces, and motion.
According to Newton's 1st Law of Motion (aka "inertia") bodies tend to keep doing whatever they are already doing!
Thus - as long as forces on it are balanced -
-----a body at rest will remain at rest, and
-----a moving body keeps moving
...........in a straight line
...........at a constant speed.
However, a NET force will cause a CHANGE in velocity, affecting speed and/or direction of the body.
Such a change in velocity is called acceleration.