The mind probe was a device that allowed a person to access the knowledge in another person's mind. It was built by Anubis using Ancient knowledge; the Ancients themselves would never have built such a device.
The mind probe was a small sphere, from which spikes extended after activation. Once the device was implanted into the prefrontal cortex of a person's brain, a link was created between the mind of the person and a computer. Afterwards the conscious knowledge could be downloaded into the memory banks of the computer.
The only know user of the mind probe was the Goa'uld Anubis, who employed it on his prisoners to obtain information. The first known person he used it onto was the Asgard Thor, from whose mind Anubis acquired the information about the Asgard's hologram and beaming technology. Although Thor was rescued by SG-1, and while the Asgard were able to remove the device from Thor's brain, it was too late, as Thor's entire consciousness had been transferred to the ship's computer. (SG1: "Revelations", "Descent")