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)
The Tenth Doctor sometimes took the shield down, at least once to listen the Music of the Spheres. This resulted in the Fifth Doctor's TARDIS to merge with the Tenth Doctor's, the Space-Titanic to crash into it, and a Graske and a time portal to gain access. (TV: Last of the Time Lords, Time Crash, Voyage of the Damned, Music of the Spheres)
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)