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.
Taken from The Mars Volta album "De-Loused From The Comatorium" and linked to the word Inertia (the feeling of not being able to do anything, like there's no will to do anything). Possibly the way you are feeling at the time.
"Dude.. I dunno.. I've just been feeling really inertiatic lately.. Like, I won't even get up in the morning.."