These settings will only work on tapes that were recorded using the corresponding type of Dolby Noise Reduction (NR). Many cassette tape machines meant to play music cassettes - will have either Dolby B or Dolby C or both settings as one of the controls available. It's a rather sophisticated technology that must be used during the recording process and played back on a machine with the proper decoder. Thanks to Dolby, the cassette tape became the most popular medium for car stereos until the CD player was released. In the 1960's Dolby Laboratories invented a complex system for reducing this hiss which allowed the cassette tape medium to render tolerable (if not quite hi fidelity) reproductions of music. Tape hiss is the high frequency noise present on analogue magnetic tape recordings caused by the size of the magnetic particles used to make the tape.
Here is a quote from Wikipedia article on 'Tape Hiss':
Unfortunately one of the biggest problems with cassette tapes using the magnetic tape format was their propensity to produce prominent hiss. Magnetic tape was one of the first formats that allowed folks to easily record speech or music at home.
