"A body persists its state of rest or of uniform motion unless acted upon by an external unbalanced force."
Newton's laws of motion
I know you want to hear this as a: "what it is in a nutshell" but if you really want more then "yes/no" just read about it and think about it; I can give you the "1" the "+" and the "2" but only you can put the "=3" in there.