`_' undeclared (first use this function) ??

["Kang, Michael" <MKang sweda com>]

Hi,

I got a compile error on the following function call.
        gtk_entry_set_text (GTK_ENTRY (combo_entry1), _("WinPst Desktop"));

I could not find the defination of _(), and I can not search it on google.
What is that? How can I get rid of the error?

Thanks in advance,

Michael