Focal plane tilt is a source of aberration (distortion) in optical instruments (such as telescopes) consisting of a focal plane that is not perpendicular to the optical axis. For a telescope symmetric around its optical axis, it represents a misadjustment or a flaw in construction. If the sensor can be aligned with this focal plane, it results in some distortion, and if not aligned, then the entire sensor cannot be focused: there will be some direction across the image in which the quality of the focus is varying. Aberrations due to atmospheric effects can result in an approximation of focal plane tilt, and one of the mechanisms to overcome this is either to adjust the sensors to match most closely, or to compensate through optics.
Focal plane tilt is sometimes deliberately used in (non-astronomical) photography, e.g., to bring your attention to one portion of the picture by giving it better focus.