The Ark was an enormous spacecraft. It was built in circa 10,000,000 to carry the entire human, Monoid, animal and plant population of Earth, which was about to plunge into the Sun, to a new home on Refusis II. Because the journey would take 700 years, all but the Guardians were miniaturised and placed in suspended animation.
The First Doctor, Steven Taylor and Dodo Chaplet visited the Ark twice, once at the beginning and once at the end of its journey. It was Dodo who gave the ship the nickname of "the Ark" on the first visit, as humanity had long since forgotten the Biblical story of Noah's Ark. (TV: The Ark)