Complex sentence: In flat spacetimes all inertial frames are in a state of constant, uniform motion with respect to each other, while all non-inertial frames are accelerating with respect to all inertial frames.
Split into simpler sentences: In flat spacetimes, all inertial frames are in a state of constant, uniform motion with respect to one another. All non-inertial frames are accelerating with respect to all inertial frames.