The Doctor deduced they were blind because they seemed to feel their way around the exterior of the TARDIS. Later, he determined that they had the ability to detect sound waves and heat. They were powered by a form of magnetism, and could be immobilized 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 roughly in half. The concentric rings of metal that comprised their bodes folded down 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 analyze 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 recipient. They continued to appear around the Drahvins' ship because the Rills were merely trying to communicate.
The Chumblies assisted the Rills in a variety of other ways. Because the Rills could not metabolize oxygen, the Chumblies were the only way in which the Rills could experience oxygen-rich planets, such as the one on which they had crashed. They were thus a kind of probe, sending a variety of data back to their masters.
Although principally tools of exploration and communication, the Chumblies also had offensive capabilities. They were equipped with variety of weapons. They had a flame thrower, and could also fire projectiles akin to bullets and ammonia bombs.
Their true, Rill name was unknown. The name "Chumbley" was in fact a nickname Vicki gave to them, based upon the "wobbly" sound they made when moving. When she told the Rills she had named the robots "Chumblies", they did not object. Indeed, they were quite happy to use the term themselves.