Keystoning is name for the visual effect of a trapezoidal image. Generally, this is caused when the projector is at an angle to the screen surface. The resulting image, instead of rectangular, has a narrow top and wide bottom (or the opposite).
Keystone Correction is the term for digitally trying to correct the effect. This always results in a loss of resolution, and often adds additional undesirable artifacts.
Proper installation of an HDTV projector should ensure its positioned correctly to the screen so such electronic adjustments are not necessary.