If you want to access it through the code-behind, then
<asp:HiddenField ID="fra1" runat="server" Value="Foo"/>
can be accessed exactly like any other ASP control. You can even change the value on postbacks.
If you are using JavaScript, your best bet would be to use
document.getElementById('elementId')
. This is one of the basic JS methods, so there are lots of references on the web.