The Terrestoids were human-like robots under the control of Madrigor, who intended to use them in his plan to conquer Gallifrey and the Time Ioniser.
They were easily deactivated by the Third Doctor's Ion Polarity Retarder. After trying to stop the Doctor and Sarah-Jane from reaching the TARDIS, they were apparently sent by the Doctor somewhere were Madrigor couldn't pose a threat. (PROSE: The Time Thief)