The "Destroyers" were the Goa'uld as referred to in a radio message from P3R-233. The planet had been destroyed by the Goa'uld and left lifeless and contaminated by radiation, marked with the sign of korush'nai. Before their demise, the inhabitants had sent a deep space transmission using a derivation of ancient Egyptian with the message, "Beware the Destroyers. They come from 3, 32, 16, 8, 10, 12." In an alternate reality, Daniel had translated the message, but it was too late to save Earth from the attack. However, he was able to return to his own reality with the coordinates for the origin of the Goa'uld attack. The coordinates were for a stargate aboard one of two Goa'uld motherships belonging to Apophis and Klorel en route to destroy Earth, but the advance warning allowed SG-1 to prevent the attack.
Cross Reference: Alternate Reality - P3R-233, Korush'nai, P3R-233, Quantum Mirror
Episode Reference: There But For the Grace of God
Stargate Coordinates for the Destroyers: 3-32-16-8-10-12