Bretonnia has been available as a multiplayer faction since day one, but once the February 28th update rolls around you'll also be able to play them in the singleplayer campaign. Much like all of the other Total War: Warhammer factions Bretonnia will come with unique Legendary Lords (King Louen Leoncoeur, Alberic of Bordeleaux and the Fay Enchantress), a bunch of quests that specifically relate to them, as well as a variety of newly updated units.
If you're wondering what all of this might look like in action, here's the recently posted in-game trailer. It doesn't feature much in terms of gameplay, but it should give you a general idea of what Bretonnia is all about. Have a look:
On a slightly unrelated note, it is also worth mentioning that the February 28th update will bring with it the official Battle Map Editor. As you might expect from the name alone, it will allow you to create any type of battle map using all of the props, enviromental models, SFX and VFX present in Total War: Warhammer. And best of all, you'll be able to use these custom maps in the singleplayer campaign!
And finally, if you've somehow landed here without actually owning Total War: Warhammer, you're in luck as the currently ongoing Humble Monthly bundle is offering it for a measly $12! Besides featuring the biggest discount for Total War: Warhammer ever, the Humble Monthly bundle will also bring with it a bunch of mystery games at the beginning of March, so I would definitely suggest you grab this one if you have even an inkling of interest in Warhammer. Just make sure to unsubscribe if you're only doing this for Total War: Warhammer!