These are form values that go in the HTTP request using the POST method. (including jQuery POST requests).
Say you did an ajax post like
$.ajax({
type: 'POST',
url: window.updatePost,
data: { id: 21, title: 'snappy title' },
//kept short for clarity
});
Here the two values in json, id and title, would be bound to the matching action, e.g.
public JsonResult UpdatePost(int id, string title) {
...
}