Complex sentence: The essential characteristic of all these circuits is they have a long time constant, much longer than a single period of the oscillation, over a single cycle they act as virtually linear elements, and so introduce very little distortion.
Split into simpler sentences: The essential characteristic of all these circuits is they have a long time constant, much longer than a single period of the oscillation. Therefore over a single cycle they act as virtually linear elements, and so introduce very little distortion.