At it's core, Friday Night Funkin' is a game where you have sick rap battles in order so that you can hang out with your girlfriend(subsequently named Girlfriend in the game). You play as Boyfriend, a blue haired 19 year old who just wants some peace and quiet with his girl, but is constantly engaged by her family and various foes in rap combat.
The game follows a pretty standard rhythm game format, sort of a cross between Parappa The Rapper and DDR. Four arrows at the top of your screen, one for each directional input (Left, Down, Up, and Right, bound to WASD or your arrow keys at time of review.) Your opponent will sing a sequence of notes, you'll repeat that sequence of notes with some slight variation, and occasionally you'll sing some backup vocals before spitting your own version of the verse.
As it's a rhythm game, the music you're listening to is incredibly important. I've gotta say this is one of the best rhythm game soundtracks I've listened to in a while. It's funky, it's fresh, it's catchy, and it doesn't get repetitive when you're trying to FC a specific track. Most of all, it's, for the most part, entirely sample based and handmade, with help from various Newgrounds creators and voice actors.
I'd like to review more of the game, but the two factors holding me back from that is the fact that:
A. It's truly an experience that defies accurate description
and B. It's just that short. At time of writing it is still mid development(They have a kickstarter you should consider donating to if you like the game
here) and only has 7 Weeks and a small handful of songs. All of them are great, but short. I greatly recommend downloading the mod that converts the keybinds to DFJK and just enjoying the experience for the time your there. Maybe try to FC a few songs.