In CRM 2011 RibbonDiffXML LabelText, ToolTipDescription, ToolTipTitle, what does $Resources reference?

Hi Experts,

In CRM 2011 RibbonDiffXML under the LabelText, ToolTipDescription and ToolTipTitle properties, what does $Resources reference?

Where can we get the values of these?

Any help or pointer would be helpful.

Thanks,

Palani