There are a couple of ways you can accomplish this...
First, on each page in the Load, you could manually add an HtmlMeta control to the Page's header...
protected void Page_Load(object sender, EventArgs e)
{
HtmlMeta meta = new HtmlMeta();
meta.Name = "Description";
meta.Content = "The description goes here...";
Page.Header.Controls.Add(meta);
}
Alternatively, you could place a ContentPlaceHolder in teh HEAD of your master page. Then in each page where you want a custom Description, you could specify an asp:content, and put your meta tag in there..
In the Master Page:
<head runat="server">
<title>Untitled Page</title>
<asp:contentplaceholder id="headerPlaceHolder" runat="server">
</asp:contentplaceholder>
</head>
Then, in your content pages (which reference the master page)
<asp:Content runat="server" ContentPlaceHolderID="headerPlaceHolder">
<meta name="Description" content="The description goes here..." />
</asp:Content>
Hope this helps...