The Tobin civilization, originating from Earth thousands of years ago, has been extinct for several centuries. Their legacy is a large orbiting minefield in space, designed to protect the planet. The hundreds of thousands of mines can be programmed to hone in on a variety of energy signatures. Symbols on the mines are similar to Phoenician letters, and translate as "Two to the blue, three to the orange, two to the blue," after allowing for a number system that uses zero.
Apophis and Heru'ur agreed to meet within the Tobin minefield as neutral territory to discuss a possible alliance, and SG-1 reprogrammed one of the mines to target Apophis's ship in order to sabotage the alliance. However, Apophis had concealed his cloaked fleet within the minefield, and instead he destroyed Heru'ur and absorbed his forces into his own.
Cross Reference: Apophis, Heru'ur, Terok
Episode Reference: The Serpent's Venom