Used in various online fiction, fan fiction, and erotica, these are short groups of abbreviations that tell the reader what the story will feature.
For example, in erotic stories found online, the code for adult male is M and the code for adult female is F, thus stories that feature a man and a woman engaging in some sort of sexual situation would initially be labeled as MF. If it were a lesbian pairing it would be FF and if it were some sort of threesome or other imbroglio, FFM or MMF would be used.
As well, the specific activities engaged in are usually noted, i.e. the code for exhibitionism is exhib, the code for oral sex is oral.
Thus, FM+, bd, fist, hist would mean a woman and multiple men participating in bondage and discipline and fisting in a historical time period.
This term is also used in fan fiction, though the names of characters or abbreviations of such are used, often separated by a slash ( / ). This is where the term "slash fiction" came into play, meaning two characters of the same sex being written about in a romantic or sexual manner.
For example in the Star Trek Original Series, K/S is short for Kirk and Spock. In another popular fandom, Buffy the Vampire Slayer, they combine the characters names as a abbreviated story code, so a story about Buffy and Spike would be catigorized a Spuffy.
A full listing of erotica story codes can be found here: http://www.asstr.org/~Uther_Pendragon/code/scfr.htm