The letter 'S' represents "Snake", and 'H' stands for "Hole" and thus SHSH is most frequently used between heterosexual couples. Permutations including "SSSS" and "HHHH" are also used.
Repetitive use of "SH" represents the degree to which one lover yearns for the other, thus "SHSHSHSH" represents much greater passion than just "SHSH."
Another interpretation of repeating "SH" characters represents a commitment of one lover to the other of promised climaxes for their next sexual encounter (e.g., "SHSHSH" represents three pledged orgasms). This interpretation is less common, however, as living up to a "SHSHSH" commitment might be difficult, and signing a letter or email with just "S", while honest, does not provide enough context to convey the proper affection.
"Snakes" and "Holes" derive from the exclamation "I'm going to put my snake into your hole!" This was first popularized in the mid-1980s by teenagers playing the Apple ] video game Snake Byte, in which one of the goals of the game is to guide a phallic, ever-growing "snake" through a "hole" at the top of the screen.