This is the length of the effective
optical path of a telescope or eyepiece (the distance from the main mirror or lens where the light is gathered to the point where the prime focus image is formed). Focal length is typically expressed in millimeters.
The longer the focal length, the higher the magnification and the narrower the field of view with any given eyepiece. The shorter the focal length, the lower the magnification and the wider the field of view with the same eyepiece.