<%$ Resources: EPiServer, admin.admingroup.deletegrouptooltip %>
What it does? It will search through language files and will find a label associated with the "admin.admingroup.deletegrouptooltip" key for currently selected language. What is so cool about that? Well ... the best thing is that you can use it with labels, as a error message for validation and so on ... let me give you an example:
<asp:Label Text="<%$ Resources: EPiServer, usersettings.username %>" AssociatedControlID="UserName" runat="server" />
<asp:TextBox ID="UserName" ToolTip="<%$ Resources: EPiServer, usersettings.username %>" runat="server" />
<asp:RequiredFieldValidator ControlToValidate="UserName" ErrorMessage="<%$ Resources: EPiServer, login.error.username %>" runat="server"/>
How can you set for example localized tool tip for a button not using that approach? I guess the only option is to do that in code behind file in a following way:
UserName.ToolTip = Translate("/usersettings/username");
It's only one additional line ... but having all settings for the control in one place gives you cleaner code, so in the end less chances that you will overlook something. Moreover if you make a mistake typing the label key, you can correct it and see the results with recompiling the code which can save lots of time.
You can find many examples of this syntax (including the one which I used here) just by checking EPiServer's Public Templates.