Hello Andrea,
Thank you for writing.
In order to add tooltips to the items in the column chooser, you can subscribe to the RadGridView.
ColumnChooserItemElementCreating event and assign the desired tooltip to the ColumnChooserItemElementCreatingEventArgs.ItemElement.
ToolTipText property:
I hope this information helps. Should you have further questions I would be glad to help.
Regards,
Dess
Telerik by Progress