What does S and M mean in the song?

S and M is a popular abbreviation for Sadism and Masochism. One partner will act as "S" and another will act as "M". The Sadist, or "S" will maintain a sexual control over the "M", the Masochist. This control may include sexual torture and/or deprivation or sexual release. Any degree of extremity may apply to this genre of sexuality, whatever the Masochist's limits may be.