If the label has been set so that it has an mnemonic key (using i.e. Label.setMarkupWithMnemonic, Label.setTextWithMnemonic, Label.newWithMnemonic or the “use_underline” property) the label can be associated with a widget that is the target of the mnemonic. When the label is inside a widget (like a gtk.Button or a gtk.Notebook tab) it is automatically associated with the correct widget, but sometimes (i.e. when the target is a gtk.Entry next to the label) you need to set it explicitly using this function.
The target widget will be accelerated by emitting the GtkWidget::mnemonic-activate signal on it. The default handler for this signal will activate the widget if there are no mnemonic collisions and toggle focus between the colliding widgets otherwise.