These were one of many types of weapons created by Ma'chello to combat the Goa'uld Empire. The Goa'uld Killers were placed within Goa'uld page turning devices when interacted with a specially made tablet. Once the page turner was used on the tablet, 10 small white grub-like worms would fall from the device. They would immediately begin to find a host body and infect it.
The Goa'uld Killers were capable of piercing through protective gloves and glass containers easily as if the obstacle was not within the way. Once they entered into the host, they would immediately determine if a Goa'uld symbiote was located within the body. Should a Goa'uld be present, the Goa'uld Killers would immediately attack the organism and kill it, in the process freeing the host from the parasites control. Once this was done, an automated message would inform the host that they were free from their captor after which the small Goa'uld killers would exit the host body from the ear and die. Somehow, the symbiote toxin released was counteracted.
However, should the host not possess a symbiote then the Goa'uld Killers can result in the organism from delusions to hallucinations that can drive the person mad. They will remain in the body until they detect a body that possesses a Goa'uld symbiote which results in the machine departing the body and infecting a new host.
Hosts that possessed the protein marker that signifies a body where the symbiote has died signals the small machines that the Goa'uld is dead after which the Goa'uld Killers deactivate. (SG1: "Legacy")
Before his death, Ma'chello placed some of these machines in a chamber that was used by the Linvris. The nine Goa'uld would accidentally activate the machine and be killed in the process leaving a single Goa'uld Killer dormant until SG-1 arrived and Daniel Jackson activated the device. The lone Goa'uld Killer would leave the machine and infect Dr. Jackson. (SG1: "Legacy")