If you’ve never paid any attention to fanfiction, like roughly 98.9 percent of the known world, you’ll need to know that “shipping” in this context is a truncated form of “relationshipping,” which is the practice of pairing off existing fictional characters in a way presumably not intended by the original creator(s) of those characters. Perhaps unsurprisingly, not all such configurations work well:
I admit some of the fan “shippings” most of them, in fact don’t really work for me, but I could see Fluttershy and Big Macintosh slowly noticing each other over the months, and somehow eventually shyly starting to date… And I think she’d be a much better match for him than Cheerilee would.)
Of course, given the preponderance of female characters on MLP:FiM or, more precisely, the dearth of male characters rather a lot of ships are written specifically for that sweet mare-on-mare action. Or something like that.
I have, you may safely assume, read entirely too much of this stuff over the past several months. Of the pairings Generally Regarded As Straight, Fluttermac (to give it its standard abbreviation) is one of the more popular ones, simply because it’s one of the more plausible ones: both of them are somewhat reticent, dutiful and, yes, shy, and there’s no reason to think that either of them is simply looking for a Good Time. (Twimac works similarly, though this is based on the notion that Twilight Sparkle is utterly clueless about matters of the heart, a characteristic with only the vaguest roots in series canon.)
Then again, you might also assume that I have written entirely too much of this stuff over the past several months. I got an actual trilogy (three installments, 40k words) out of what originated as a human/pony pairing.
(Aside: Anent said trilogy, I mentioned to a friend earlier this weekend that I’d written a human/pony romance. “Which pony?” she asked. Upon being told that it was Twilight Sparkle, she said simply “Of course.”)
A later 10k-word effort set up a chance meeting between Twist, one of the grade-school fillies in the series, and a colt (an original character of my design) whose family vacation brought him to Ponyville. “Half a lifetime later,” says the pitch, “she still haunts his dreams.” Anyone who knows me might legitimately wonder why I didn’t write that one first.