I don't understand what this line ds.Tables[3] means.

  Can somebody explain to me what ds.Tables[3] in line 9 means? If you know a good link on this topic, please tell me the link. Thank you. 1 protected void bindData()
2 {
3 XmlTextReader reader =
4 new XmlTextReader(Server.MapPath("articleAtom.xml"));
5
6 DataSet ds = new DataSet();
7 ds.ReadXml(reader);
8 // Assigns the datset to the datagrid
9 GridView1.DataSource = ds.Tables[3];
10
11 // Binds the datagrid
1 ...