... though yeah, PSA for those who don't know (probably no one, but it's not a bad thing to be sure), we were talking about common shorthand for internet-published content. It's easily most prevalent in fan and original fiction (and, by extension, similarly prevalent in written adult material, which is where wierd's comment about erotica came from), but it's used to some degree for just about everything floating around that can be so flagged. There's fandom or interest specific stuff, but there's also a number of acronyms and abbreviations that are pretty ubiquitous.
In the case of the current discussion, they're common ones: You'd be looking at two works (well... probably, it's not like there's a tag limit, very often), one with content that included a polygamous relationship with two female and one male characters, that had pregnancy as a main theme or supplemental aspect (FFM Preg), and another than involved a homosexual relationship between two women and had male pregnancy as a similar theme or supplement (FF Mpreg). As noted, something of a difference :V
There's no actual certainty either would involve anything explicit or even sexually oriented, but it is good to remember stories with this sort of shorthand are definitely more likely to; they indicate that, for whatever reason, the author (or person otherwise discussing the work) felt the need to inform potential consumers (in the non-monetary sense, usually) about certain aspects of the content in the work, either as forewarning or temptation (or both, of course). Some extent of lasciviousness is one of the most common reasons for that, but not nearly the only one.
In any case, it's not difficult to find lists explaining what is what if you're interested and previously unaware, though you'd probably be best off cross referencing a few to get a good feel of things and an idea of how mutations tend to manifest. It's good information to know if you're starting to make (serious) forays into the great webby wilderness, or find yourself becoming interested in the creative works found online.