messages.add(message);什么意思？？？

<%@ page language="java" contentType="text/html; charset=UTF-8"
   pageEncoding="UTF-8"%>
<%@ page import="java.util.*,model.*"%>

<%
String title=request.getParameter("title");
String content=request.getParameter("content");

Message message=new Message();
message.setTitle(title);
message.setContent(content);

List<Message> messages=(List<Message>)session.getAttribute("message");
if(messages==null){
messages=new ArrayList<Message>();
session.setAttribute("messages", messages);

}
messages.add(message);//这一句什么意思？？？
response.sendRedirect(request.getContextPath()+"/message.jsp?subFlag=1");


%>