Down near the submit buttons? Its a preview

Next to the "post comment"? It lets you see how the post will look like, it's especially useful If you are adding an image/gif/youtubevid it lets you see how it'll look with it

This is what happens when you click on it.

You can see how your message is.

The eye one shows you what the post will look like before you submit it. It helps when posting pics or youtube videos.

It lets you view your post before you post it

Respond to your messages.