It was a hand-sized device. When activated, it emitted a burst of resonating sounds. It was also capable of opening doorways within Silurian bases.
The Silurians gave a summoning device to the human scientist Dr Quinn to recover a wounded and lost member of their race being hunted by humans on the surface. After Dr Quinn was killed when he tried to imprison the Silurian, the Third Doctor took the device from Quinn's body and used it to enter the Silurian base in the caves. (TV: Doctor Who and the Silurians)