how AutoCompleteExtender works

<asp:TableCell HorizontalAlign="Left">
                           Subject Name:<asp:TextBox ID="txtSubject" runat="server"></asp:TextBox><img id="loading" style="visibility:hidden" src="~/Images/Wait.gif" />
                           <ajax:AutoCompleteExtender ID="AutoCompleteExtender1" EnableCaching="true"
                               BehaviorID="AutoCompleteEx" TargetControlID="txtSubject"
                               ServiceMethod="Getsubjects" MinimumPrefixLength="1"
                               CompletionSetCount="1" CompletionInterval="1000"
                               runat="server" FirstRowSelected="true">
                           </ajax:AutoCompleteExtender>
                           <!-- <asp:DropDownList ID="ddlSubject" runat="server" AutoPostBack="True" OnSelectedIndexChanged="ddlSubject_SelectedIndexChanged"
                       Width="140px" CssClass="asp_dropdown">
                   </asp:DropDownList>-->
                       </asp:TableCell>

[System.Web.Script.Services.ScriptMethod()]
    [System.Web.Services.WebMethod]
    public List<string> Getsubjects(string prefixText)
    {
        DataSet ds = new DataSet();
        ds = PopulateSubjects(prefixText);
        DataTable dt = ds.Tables["ResultSet"];
        List<string> SbujectNames = new List<string>();
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            SbujectNames.Add(dt.Rows[i][1].ToString());
        }
        return SbujectNames;
    }