What is Instagram's "code" and "redirect url"?

I'm developing an application with Instagram integration. For this I use the GitHub project jInstagram (https://github.com/sachin-handiekar/jInstagram).

If you want to log in to Instagram you have to set a redirect url. My first question is: What is this URL for? I didn't find detailed information about this. The code in jInstagram looks like this:

InstagramService service = new InstagramAuthService()
                        .apiKey("your_client_id")
                        .apiSecret("your_client_secret")
                        .callback("your_callback_url")     
                        .build();

Source: https://github.com/sachin-handiekar/jInstagram/wiki/jInstagram-Usage

To get the access token I have to create a verifier and generate an access token from this. In jInstagram's Usage the only information about this verifier is this:

Verifier verifier = new Verifier("verifier you get from the user");
Token accessToken = service.getAccessToken(EMPTY_TOKEN, verifier);

Source: https://github.com/sachin-handiekar/jInstagram/wiki/jInstagram-Usage

What do I have to pass to the verifier's constructor? I searched for an answer and discovered that I have to pass a request parameter named "code" in the code example: https://github.com/sachinhandiekar/jInstagramexamples/blob/master/src/main/java/com/sachinhandiekar/examples/InstagramTokenHandler.java

    private static final String CODE = "code";

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws javax.servlet.ServletException, IOException {
    String code = request.getParameter(CODE);

    InstagramService service = (InstagramService) request.getServletContext().getAttribute(Constants.INSTAGRAM_SERVICE);

    Verifier verifier = new Verifier(code);

    Token accessToken = service.getAccessToken(verifier);
    Instagram instagram = new Instagram(accessToken);

    HttpSession session = request.getSession();

    session.setAttribute(Constants.INSTAGRAM_OBJECT, instagram);

    System.out.println(request.getContextPath());
    // Redirect to User Profile page.
    response.sendRedirect(request.getContextPath() + "/profile.jsp");

}

What is this code? How do I get in possession of it?

Thank you for reading this question. I'm looking forward to get answers.