A Goa'uld homing device is a tracking machine designed by the Goa'uld.
A homing device is designed to find and give direction even to the distance to a target. The viewfinder is presumably powered by naquadah.
In 1997, Teal'c mentioned Goa'uld use of homing devices on their behalf. He also declared that when he was on the Gaia first time, he and his men did not bring any homing devices on their mission. (SG1: "The Nox")