Tardis stands for Time and Relative Dimension (or Dimensions) in Space. It's the Doctor's machine for travelling through time and space.
It's also his home. The strange thing about the Tardis is that it's bigger on the inside than it is on the outside.
The Tardis once had the ability to change shape to blend in with its surroundings, but its "chameleon circuit" broke.
It is now stuck in the shape of a blue police box - which was a common sight in Britain in the mid 20th Century.