The contents of the label.
If the string contains Pango XML markup, you will have to set the use_markup property to true in order for the label to display the markup attributes. See also set_markup for a convenience function that sets both this property and the use_markup property at the same time.
If the string contains underlines acting as mnemonics, you will have to set the use_underline property to true in order for the label to display them.