The A't'trr are a race of sentient microscopic organisms. Not much is known about their species, other than they possess advanced technological knowledge, and possess a hive mind that allows them to act as one.
About 100 millennia ago, their homeworld P5C-353, began to die, leaving its population unable to live on the planet any longer. For this reason, a stasis device; "The Orb", was created, which allowed the A't'trr survivors to preserve themselves in storage. On the Orb, it was written, in their own mysterious language, that they hoped a species exploring their world would come through the Stargate and take them to another world; where they would go forth and multiply.
This finally came to pass when SG-1, investigating a series of energy readings, tracked the Orb down, hoping to secure a long-lasting power source for Earth's usage. It remained in quarantine at Stargate Command, until, after confirming it was now in a stable atmosphere, began to activate, provoking Colonel Jack O'Neill and Teal'c to attempt to return it to its dead world.
Unable to return without dying, the Orb secured itself in the Embarkation room, pinning O'Neill to a wall with protruding spikes. Teal'c's attempts to free his commanding officer and friend with his Staff weapon only resulted in the A't'trr gaining the energy necessary to begin to spread, causing Major General George S. Hammond to enact the Wildfire quarantine protocol.
Initially, the base personnel treated the Orb as hostile, making repeated attempts to remove and deactivate it (which all failed), until Dr. Daniel Jackson realized the A't'trr were trying to communicate. Deliberately increasing the aliens' source of energy and fuel, the aliens were able to reach critical mass inside O'Neill, and express their intelligence through his voice.
The aliens and the base's commanders were able to reason and compromise with each other before the base's self-destruct could go off (an event which would have only nourished the A't'trr the energy to spread over the face of the Earth and consume it): the Orb, along with the A't'trr, would be sent to P4G-881, a primordial planet with an ideal environment for which they could live off. (SG1: "Message in a Bottle")