The fifth and sixth ecumenical councils had both been convened to settle doctrine and both had done exactly that and nothing else. Neither issued disciplinary canons. That left the eastern church with two councils of the highest authority and no new church law from either, at a time when practice varied widely across the patriarchates and the older canonical collections were centuries out of date.
The council of 692 was called to fill the gap. Its name says so: quinisext means fifth and sixth together, and it met in the same domed hall of the imperial palace as the sixth had, which is why it is also called the council in Trullo. It issued a hundred and two canons and claimed the authority of the two councils it was completing.
The canons are a portrait of what the eastern church was actually worried about. Clergy discipline and the terms on which a married man may serve. What may be sung, and by whom. The regulation of festivals inherited from pagan practice. Rules against gambling clergy, against clerics keeping taverns, against a range of local customs the council wanted stopped. Provisions on marriage, penance and the treatment of those returning from heresy.
For the Orthodox churches this is church law and has been continuously since. The canons are cited, interpreted and applied, and a good deal of what an Orthodox parish looks like today can be traced to them.