Refers to the exact pitch, an A, that a modern orchestra aspires to tune its instruments to. The 440 is for 440Hz, which (in my limited understanding of physics) is the frequency at which this particular sound wave oscillates. While this is sort of the standard A these days, A is recognizable over a wide range of frequencies. For a nice list of different historical As see http://www.uk-piano.org/history/pitch.html.