What exactly is the inertial frame of reference?

the inertial reference frame is not absolute. because motion is not absolute. motion is relative depends on who's looking/observing.

An inertial frame of reference (FOR) is a non-accelerating FOR , for example if a person is observing a moving car while at rest or while moving at constant velocity, he is in an inertial FOR. A...

An inertial frame of reference = constant vel. non inertial frame of reference = acceleration

Because we're rotating with it

This conclusion was reached as a result of many experiments, both thought experiments and physical ones. The most famous one is in the measurement of aether velocity by Michelson Morley. If the speed...