A force is a vector, with both a magnitude and a direction.
The MKS unit of force is the newton (N). 1 N = 1 kg m / s2.
The tendency of an object to maintain its state of motion is known as inertia. Mass is how we measure inertia for straight-line motion; light objects are easy to move, but heavy objects are much harder to move, and it is much harder to change their motion once they start moving.
The Greeks (Aristotle, etc.) thought that the natural state of motion of an object is at rest. Newton's first law is a much more sophisticated view of motion.
Isaac Newton (1642-1727) studied forces and noticed three things in particular about them. These are important enough that we call them Newton's laws of motion.
Do Newton's laws apply all the time? As long as we're in a stationary reference frame, or even moving at constant velocity, Newton's law are valid. Such reference frames are called inertial reference frames.
Newton's Laws are not enough to account for motion observed from non-inertial (accelerating) reference frames.