They were created by and under the control of the Rills.
The Doctor deduced they were blind because they seemed to feel their way around the exterior of the Doctor's TARDIS. Later, he determined they had the ability to detect sound waves and heat. They were powered by a form of magnetism and could be immobilised by ensnaring them in a metal mesh. They resisted gunfire by voluntary deactivation. When they switched themselves off, their bodies telescoped down, reducing their height by roughly half. The concentric rings of metal that comprised their bodies folded into each other, thereby protecting the interior of the robot.
Their primary task was to act as a translator between the Rills and other beings. They could analyse the speech patterns of other beings and allow the thoughts of the Rills to be "spoken" through their onboard speakers in the language of the listener. (For example, the Doctor, Vicki and Steven heard the Rills' thoughts as though the Rills were "speaking" English.) They continued to appear around the Drahvins' ship because the Rills were merely trying to communicate.
The Chumblies assisted the Rills in other ways. Because the Rills could not metabolise oxygen, the Chumblies were the only way the Rills could experience oxygen-rich planets, such as the one on which they had crashed. They were thus a kind of probe, sending data back to their masters. (TV: Galaxy 4)
Although principally tools of exploration and communication, the Chumblies had ways of attacking. They were equipped with weapons. They had flame-throwers and could also fire projectiles akin to bullets and ammonia bombs. (TV: Galaxy 4)
Their true Rill name, if any, was unknown. "Chumbley" was what Vicki called them, based upon the "wobbly" sound they made when moving about.
When Vicki wanted to know whose voice she could hear speaking to her in the Rill central chamber ("Look, who is talking? Is it... this Chumbley, or is it... someone else?"), the Rills asked "You call the robots Chumblies?" and when Vicki confirmed this, they did not object. Indeed, the Rills used the term themselves, explaining to Vicki "The Chumblies have a speaker in them. They are transmitting our thoughts." (TV: Galaxy 4)
Behind the scenes
- According to Jeremy Bentham, the Chumblies were designed by Richard Hunt. Like Daleks, they required a performer to operate the machinery from inside. However, their small, three-foot stature meant that those performers all had to be very small. The action of the Chumblies telescoping down into a "closed" position when attacked or deactivated made the prop too small for any performer. A "performer-less" dummy prop was constructed for filming the "down" position.