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The term fiber positioner can refer to a mechanical instrument that positions optical fibers (long strands of glass which can carry light) for use in a multi-object spectrograph, placing it so that light from the image of a particular object on the focal plane is carried to a spot along the line in a spectrograph where the light is split by wavelength, i.e., the spectrograph's slit (or its line corresponding to such a slit). Thus, objects spread out over the image at the focal plane are lined up in a row so that a single spectrograph can spread their spectrums, resulting in a bunch of spectrums laid out in parallel, which a CCD or similar sensor can record.
The task of positioning the fibers has been carried out by hand, which is time-consuming, requires very precise work, and spectrographs now exist that can observe hundreds of objects simultaneously, demanding a lot of such work for each such observation, making mechanical positioners a natural development, given current technology. The APOGEE spectrograph used by the Sloan Digital Sky Survey originally depended upon staff to position fibers, using pre-created aluminum plates with carefully positioned holes (plug plates, which were created using automated hole positioning). The spectrograph's fiber positioning has since been automated. Some other robotic (i.e., automated) fiber positioners:
Naturally the term fiber positioner could also refer to a person tasked with the positioning (like the word computer, which earlier referred to a person who calculates, by hand or with a calculator). Outside outside astronomy, fiber positioner can refer to devices that line up optical fibers end-to-end, such as for communication.