Gtk.CellRendererCombo renders text in a cell like Gtk.CellRendererText from which it is derived. But while Gtk.CellRendererText offers a simple entry to edit the text, Gtk.CellRendererCombo offers a Gtk.ComboBox or Gtk.ComboBoxEntry widget to edit the text. The values to display in the combo box are taken from the tree model specified in the model property.
The combo cell renderer takes care of adding a text cell renderer to the combo box and sets it to display the column specified by its text-column property. Further cell renderers can be added in a handler for the editing-started signal.