Simple terms: The 'reason' that to why you have to use some sort of effort to move or stop something like an object.
The law/principle of inertia states that a body at rest will remain at rest and a body
in motion will continue moving with a constant velocity unless acted upon by an external
non zero force.
Don't confuse this with
momentum
Due to inertia, a moving body
in space (ie a vacuum) will not stop or
deviate unless another
non zero force acts on it (eg gravity)