Ejection bubbles were, according to the Fourth Doctor, "a space parachute".
When a Mutt cruiser was passing by Karn, the Sisterhood of Karn caused the cruiser to crash on the surface. One of the occupants, Kriz, managed to escape in an ejection bubble, but was soon killed by Condo for his head. After the Doctor's TARDIS landed on Karn, the ejection bubble was found by Sarah Jane Smith. (TV: The Brain of Morbius)