SG-1 first encountered alternate realities with the discovery of the quantum mirror on P3R-233. It is a portal to any number of alternate universes, and through it, Daniel was inadvertently transported to a reality in which Earth was under attack by the Goa'uld.
There were similarities in this version of Earth. O'Neill had led the suicide mission to Abydos to blow up the gate and destroy Ra, his son had accidentally killed himself with O'Neill's gun, and Teal'c had a wife and son on Chulak. However, on this Earth, Stargate Command was known as the SGA, O'Neill was a brigadier general, Hammond a colonel, Carter a civilian with a Ph.D. in astrophysics, and O'Neill and Carter were engaged. Catherine Langford had approached Daniel years before, but he had rudely refused to join the team translating the Giza cartouche, and he had probably been killed in Egypt during the recent Goa'uld attacks. It was Catherine who had supervised the translation team, and she had remained with the stargate project after the initial mission. There had been no mission to Chulak, instead, a bomb had been sent through the gate to destroy Chulak, and Teal'c had remained the First Prime of Apophis, leading the attack on Earth. Already over 1.5 billion people had died, and refugees from the Genesis list, including world leaders, scientists, and doctors, were being evacuated to the Alpha Site, known as the Beta Site in that reality.
Earth would eventually be conquered by the Goa'uld, but a radio message held the key to preventing the same fate for the Earth of Daniel's reality. Three months before his arrival, a deep space transmission from P3R-233 used a derivation of ancient Egyptian to send the message: "Beware the Destroyers. They come from 3, 32, 16, 8, 10, 12." These coordinates for the origin of the Goa'uld attack were brought back by Daniel through the mirror, and they enabled SG-1 to prevent the same fate for Earth in their own reality.
Cross Reference: Alternate Reality, Alternate Reality - Quantum Mirror, Alternate Reality - Ripple Effect, Alternate Reality - Universal Bridge, Beta Site, Destroyers, Catherine Langford, P3R-233, Quantum Mirror
Episode Reference: There But For the Grace of God