The inertial frame of reference is any frame of reference in which objects exhibit inertia alternatively it can be defined as any reference frame in which the origin is not accelerating or rotating.

In this frame observations can be made without the need to correct for acceleration. In contrast in a non-inertial frame of reference the interactions between objects change with acceleration and must be accounted for.

Usually because acceleration is impossible to determine objectively (as all objects in our galaxy are being accelerated by some force) a distant star is used as to approximate the base inertial frame of reference to compare all others to.