The symbiote poison is a chemical compound designed to attack Goa'uld and Tok'ra symbiotes. Composed of two liquids, when mixed together, they create a deadly gas that causes almost instantaneous death to the symbiote. It will also work on Jaffa, since they depend on their larval symbiote to survive. While not fatal outright to the host, the dying symbiote will automatically release a toxin into their host, killing it.
The Tok'ra developed it to assassinate the System Lords while the Goa'uld Empire was in chaos following the deaths of Apophis and Cronus, but they were forced to abandon it when they learned that Anubis was alive and would thus take over any assets left by the System Lords. Lt. Kevin Elliot and the Tok'ra symbiote Lantash who was using him as a host proceeded to use the poison to take out the army of Jaffa guarding the Revanna Stargate during the Battle of Revanna to save SG-1 and Jacob Carter/Selmak. Both died in the process from the poison, but as Elliot was mortally injured with Lantash too weak to heal him, they willingly sacrificed themselves to save everyone else. The formula was given to the Tau'ri, who made a more potent version with properties similar to nerve gas.
Later, The Trust gained possession of a large amount of the poison and utilized it to attack Goa'uld worlds by firing missiles filled with the gas through the Stargate, which they had recently stolen. SG-1 was able to reacquire the Stargate, but the Al'kesh it was on fled to hyperspace, taking enough symbiote poison with it to poison three worlds.
In 2007, Ba'al used it to kill all but three of his clones: the three Ba'als that survived were the original one that was subsequently killed by an alternate version of Cameron Mitchell, a symbiote clone that was implanted into Adria but removed by the Tok'ra and presumably killed, and the final clone that was captured by the Tau'ri and turned over to the Tok'ra for symbiote extraction, killing the last Ba'al and leaving the clone host as all that was left of him. (SG1: "Summit", "Last Stand", "Covenant", "Endgame", "Dominion", "Continuum")