When Daniel activated the quantum mirror on P3R-233, he was transported to an alternate reality in which Earth was under attack by the Goa'uld. Nearly 1.5 billion people had already been killed, and refugees from the Genesis list, including world leaders, scientists, and doctors, were being evacuated through the stargate to the Beta Site as a precaution in the event of global annihilation. The Beta Site, located on P3X-984, was known as the Alpha Site in our reality, and its stargate coordinates were identical.
When Daniel returned to his own reality, he joined SG-1 on the mission to prevent a similar Goa'uld attack on Earth, and as Apophis's motherships approached Earth, refugees began to evacuate to the Alpha Site. Daniel was gravely wounded aboard Apophis's mothership and was unable to use the onboard stargate to dial Earth because its position in Earth's orbit gave it an address identical to Earth's. However, Daniel recalled the coordinates to the Beta Site, and he dialed P3X-984 instead. Just as Apophis's mothership was destroyed, Daniel was transported to the Alpha Site, from which he could dial home.
Cross Reference: Alpha Site, Alternate Reality - P3R-233, P3X-984
Episode Reference: There But For the Grace of God, The Serpent's Lair
Stargate Coordinates: 29-5-36-6-23-?