Below the Periodicity drop-down box is a field that allows the user to specify time periods that should not count as a time stamp increment with respect to the modeling, forecasting and visualization process. For example, consider daily trading data for a given stock. The market is closed for trading over the weekend and on public holidays, so these time periods do not count as an increment and the difference, for example, between market close on Friday and on the following Monday is one time unit (not three). The heuristic used to automatically detect periodicity can't cope with these "holes" in the data, so the user must specify a periodicity to use and supply the time periods that are not to considered as increments in the Skip list text field.
The following screenshot shows the default evaluation on the Australian wine training data for the "Fortified" and "Dry-white" targets.