Hello,

It means that it is the refractive index at 20 degrees (the notation may be seen as incomplete, as no mention of whether this is Celsius or Fahrenheit scale is given, but in most cases this refers to the Celsius scale) for monochromatic light corresponding to the D-line of sodium (this refers to the centre of the dublet, roughly at 589.3 nm).

I've searched a bit and found that the CRC Handbook of Physics and Chemistry (accessible online via http://www.hbcpnetbase.com/ ) supposedly contains the refractive indices for a large number of systems.

Thanks Tomasz Jarosz,