The TARDIS force field was a force field surrounding the outer plasmic shell of the craft. It protected the occupant(s) from harm (though it was far from invulnerable) and from the Time Vortex and other environments dangerous to its pilot.
During his fourth incarnation, the Doctor reconfigured the force field of his TARDIS, a Type 40, into a corridor through which he could safely traverse a vacuum. (TV: The Horns of Nimon) In his eleventh incarnation, he repeated this operation to permit River Song to transfer from another ship in space. (TV: The Time of Angels)
After the Last Great Time War, Jack Harkness and the Ninth Doctor wired a tribophysical waveform macro-kinetic extrapolator into the TARDIS to enhance or enable its force field. (TV: The Parting of the Ways)