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A reference frame is a point of view, a perspective for making observations and judgments. Special relativity is intended to apply only to inertial reference frames. Inertial frames are reference frames in which Newton's first law of inertia holds: any object's acceleration is zero if no net force acts on the object. In other words, if no unbalanced external forces are acting on a moving object, then the object moves in a straight line. It doesn't curve or go into orbit. And it travels equal distances in equal amounts of time. Any frame of reference moving at constant velocity relative to an inertial frame is also an inertial frame. A reference frame spinning relative to an inertial frame isn't an inertial frame. The presence of gravitation normally destroys any possibility of finding a frame that is a perfect inertial frame, but a reference frame in which the 'fixed' stars are at rest is approximately an inertial reference frame, as is any reference frame moving at constant velocity with respect to the 'fixed' stars. Is a reference frame attached to Earth an equally good approximation to an inertial reference frame? Not quite. The frame is spinning relative to the heavenly bodies; and the gravitational forces due to the Moon, Sun and planets will make Newton's law fail; but for many situations these influences are negligible, and computations using special relativity or even Newton's mechanics give fine results.
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