The bubble universe was a tiny universe which was home to the sentient planetoid known as House. House lured Time Lords from the main universe through the bubble universe rift so it could consume their TARDISes. The Doctor was drawn to the bubble universe by a distress hypercube from the Corsair. (DW: The Doctor's Wife)
The "bubble universe" got its name from the Doctor's description of it as "a big soap bubble with a little bubble on the outside", even though he goes onto say "well, it's nothing like that."