"Simulation" should not be a four-letter word. Just because Proteus happens to communicate the joys of exploration and nature through a digital medium doesn't mean that it has any less merit than taking a walk in the real outdoors. Still, it is impossible to deny that it is a separate experience and akin to a poem in its presentation and tone.
The game sets you down just offshore of a relatively small and uninhabited island with a variety of flora and fauna. You can walk around and observe your surroundings. There is no obvious objective or even an air of secrecy about the whole thing – it is purely a game that exists to let you wander. Eventually, you'll find out how to change the seasons and find certain landmarks that trigger heightened moments of reality. You can choose to investigate any of these moments for as long as you choose to let them linger, or let them fade away by wandering over to another point of interest.
It is in these specific visuals of nature afforded you that set Proteus apart from a walk you might experience in real life; Proteus is an artist's rendition of a walk. In the end, my experience playing this game does boil down to that possibly vague distinction, but if you're someone who likes stumbling across that one huge tree, or seeing clouds drift overhead, or eavesdropping on the smallest sounds of nature, I'd highly recommend looking at Proteus. It is a refreshing change of pace to take a walk in someone else's walk.