This means the lens has a 50mm focal length and a maximum aperture of f/1.8. Apertures are described in f/stops, which is a fractional way of indicating “lens speed,” or what the ratio is between the focal length and the diameter of the lens’s entrance pupil. In simple terms, the lower the f/number, the faster the lens is.
https://www.bhphotovideo.com/c/browse/Lenses/ci/15492/N/4288584250

