The Lighthouse is not your typical movie. For one thing, it was filmed on 80 year old lenses, and shot in black and white. Also, the aspect ratio is something you'd expect on square TVs rather than in a modern movie theater or widescreen TV. But all of these things add, not subtract from the movie. They help place you into the environment that our two characters are in. It boxes you into the movie, in the same way these characters are boxed into their island.
The movie itself follows two characters effectively losing their minds. Do some reading online, and you'll see there's quite a few ways to interpret the movie. Generally, that kind of thing can mean that the movie didn't have much to say. That is absolutely not the case for this movie. An "answer" isn't required, nor intended to exist. It is, quite simply, two characters going absolutely nuts. And boy does it lead to some great content.