The Chumblies are the robotic servants of the Rills and although they appeared menacing when the Doctor and his friends initially encountered them, they proved to be helpful and effective machines.
Their primary function was apparently to help their masters communicate with alien species and they seemed to have telepathic capabilities, able to articulate the Rills' thoughts. But Chumblies also acted as probes, gathering and relaying data on planets which their makers could not easily explore.
The Doctor's companion, Vicki, called the machines 'Chumblies' but their original name remains unknown. Although they serve the peaceful Rills, the Chumblies are able to defend themselves with in-built weaponry, so if you come across a Chumbly on sentry duty, watch out!