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A focal plane is a flat location within an optical system at which an image is focused. If something flat is placed to coincide with the focal plane, the image is formed on it: e.g., photographic film, a CCD, or a screen for viewing, or within the eye, the retina. For a telescope, the focal plane is a (mathematical) plane on which light from each point in the object observed that entered the telescope is brought back to a corresponding point, i.e., the telescope produces an image. The focusing can be accomplished with a lens or, as in a reflector telescope, a concave mirror. In the simplest case, the distance from the lens or mirror to the focal plane is termed the focal length. In this case, the telescope's focal point is the point on the focal plane to which light parallel to the optical axis is focused. Note that the optical path of the telescope can have more than one focal plane, i.e., light may be allowed to continue (no film or CCD at the focal plane to block it) and a subsequent mirror or lens could focus the light again.
No optical system produces a perfect image, one cause of imperfection (i.e., aberration) being that the focal plane (in the sense of "the surface where the image is actually focused") may be close to flat (planar) but not completely. A reason for the complexity of optics is minimizing this. (A possible method of compensating would be a sensor that is slightly curved to more closely match the focus; unfortunately this is overly difficult for current technology such as CCD chips.)
In a research telescope with multiple instruments, one may be selected by moving it into the optical path, specifically placing it at a focal plane. Possibly the instrument will have further optics to further adjust the image, producing a subsequent focal plane for processing. For example, a spectroscope may provide a grating, with the light divided by wavelength focusing on a subsequent focal plane. Furthermore, with the use of partially reflective mirrors or prisms, light may be split into multiple optical paths leading to separated focal planes, so as to process the image in different ways.