Inertia is the tendency of a mass to stay at its velocity, until it is acted upon by another force. If the mass is at rest, it will stay that way until something propels it. If it is moving, it will keep moving until something stops it; that "something" can be as simple as friction with the surface it is traveling on, the air molecules it is encountering.
The first law of inertia or the law of dynamics is state as any body preserves its state of relative rest or uniform rectilinear motion so long as the influence of other bodies doesn't compel it to change this condition.This principle can not be verified through direct experiences, because we can not isolate a body from other bodies action, just to let us verify the validity of this principle. Newton, when he formulated this principle, has considered that the movement is operating in an absolute space, immaterial one. The idea of breaking space is a matter of metaphysical conception, which does not correspond with reality. Space does exist in an objective way, and its properties are determined by the matter. The accuracy of the first principle of dynamics shows that good results are obtained as a result of using this principle. The first principle is valid only in the so-called inertial reference system,meaning a system which is either resting or moving with constant speed towards the fixed stars. Such a system could be a system with the sun in the center and axes directed towards the fixed three stars, any other RS being a neinertial one.
Inertia is the property of matter (in form of an object or a body) to remain in state of rest or uniform motion unless acted upon by an external force.
This means that an object which is motionless will continue to remain motionless unless an external force is applied to it. Similarly, an object which is moving at a constant speed in a straight line will continue to move in at the same speed and in same direction unless an external force is applied to it.
To what extent the external force will change the state of the object will depend on mass of the body, the amount of force and the period, for which the force is applied.
Application of external force to a body at rest will result in putting it in motion. The result of application of external force to a body in uniform motion will be to change the speed, or the direction of motion, or both depending upon the direction of force applied in relation to the direction of motion.