A radioactive isotope injected into the body allows one to remain undetected by Goa'uld sensors. Daniel and Jonas were injected with the isotope before infiltrating Anubis's mothership, and Carter, Teal'c, and Jacob used the isotope to allow them to move freely inside Anubis's base on Tartarus without detection. The effects of the isotope are temporary and will wear off after approximately eight hours.
Cross Reference: Anubis's Superweapon, Tartarus
Episode Reference: Fallen, Homecoming, Evolution