For these things, Google was invented.
A quick Google search pulled up a Wikipedia page explaining it.
"The sound recording copyright symbol, represented by the graphic symbol ℗ (a circled capital letter P), is the copyright symbol used to provide notice of copyright in a sound recording (phonogram) embodied in a phonorecord (LPs, audiotapes, cassettes, compact discs, etc.). The use of the symbol originated in United States copyright law and is specified internationally in the Convention for the Protection of Producers of Phonograms Against Unauthorized Duplication of Their Phonograms."
Took me less than 10 seconds.