it's a 'survival' game. you have a disease (seems like it could be a world-wide thing), and you have to find a cure. you're on an island, where scientists were working on a cure (but all ended up murdered), and you have to search for clues and ingredients (mushrooms, flowers, etc). at first you're free to derp around, but eventually a monster starts stalking you. I don't wanna spoil it, but
there's also a focus on orienteering, and marking points on your map by triangulating known landmarks. it's pretty cool
it also has a cool 'momentum' system that makes it control different from other fps's. don't expect to run and stop on a dime...
it definitely lacks some polish. a recent update fixed a lot of stuff, but it's still kinda clunky. the whole game (including the engine) was made by two brothers
here's an IGN video on it:
Miasmata - IGN Gameplay Commentary - YouTube