Ion cannons were powerful Tollan weapons used for planetary defense against an incoming orbital attack.
While the Tollan are not a militaristic people, they are also not pacifists and will destroy any vessel that poses a threat to their planet. Tollana was protected by a planetary network of cannons, which automatically tracked and destroyed threatening ships in orbit. They could also be operated manually, using a small control panel on the side of the weapon. To effectively protect a planet the size of Earth a network of 38 cannons would be needed.
These giant weapons fire charged particle streams into space from their location on the surface of the planet with deadly accuracy and were able to destroy Ha'tak vessels in one or two shots until Anubis used his ascended knowledge and Ancient technology to upgrade his Ha'tak shields, making them immune to the ion cannons.
The targeting ranges of these weapons had intentional overlapping fields of fire, so that they could provide additional protection in the event that a single cannon was either disabled or destroyed. Zipacna circumvented this by targeting all the platforms and destroying them simultaneously. However Lya of the Nox and Teal'c stopped this tactic by hiding one weapons platform which was then used by Teal'c to destroy the attacking Ha'tak. (SG1: "Pretense")
Although the SGC was left in the dark that Anubis had been responsible, Teal'c noted that the Goa'uld don't share technology with each other, and thus it was concluded that if there was an upside, it was only the perpetrator that had upgraded shields. (SG1: "Between Two Fires")