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")
While these weapons were originally able to destroy a Ha'tak vessel in a single shot, discouraging any attempts of attack by the Goa'uld, they were rendered useless after Anubis was able to tune his shields, making them impervious to the ion cannons and effective to similar particle beam technology, with the limited Ancient knowledge he possessed when he was an ascended being before he was de-ascended by the Others. They were used one last time by Narim to destroy the new phase-shifting weapons so the Goa'uld couldn't get them. (SG1: "Between Two Fires")