Question

what's inertial frame of reference ?

Answer 1

An inertial frame of reference is a point of consideration using which one can decide whether the system of particles or particle is at rest or in motion.
There (practically) exists no inertial frame of reference. As every object is in motion with respect to some other object. Example. Earth is in elliptical motion with respect to sun. But may (is) be in some weird motion with the centre of galaxy.

Answer 2

it is a frame in which newtons laws of motion are valid

Answer 3

jason's statement is the right one.