The name given to the group of ponies who include: Twilight Sparkle, Rarity, Apple Jack, Pinkie Pie, Rainbow Dash and Fluttershy; the 6 main characters of the show My Little Pony: Friendship is Magic.
Main is spelt as Mane because most names in My Little Pony are a play on words with relation to ponies or horses. So the word mane is used not only to describe the 6 main characters of the show but also is the hair on the heads and back of the necks of ponies/horses.
"OMG I just love the mane 6 in MLP"
Person 1: "So who is your favourite pony in MLP:FiM?"
Person 2: "I don't have a favourite; I love the mane 6!"