(written from a Production point of view)
According to a scene cut from the final version of "Tears of the Prophets", the Torg'Q system was a system along the Cardassian border which Worf briefly suggested as the first target in the planned offensive into Dominion space.
Off all potential targets along the border, the Torg'Q system was closest to Cardassia Prime. It was however also the most heavily fortified, and casualties were expected to be very high even though the element of surprise might come into play. Eventually the participants in the meeting choose to invade the Chin'toka system, as is seen in the episode.