If you didn't measure that morning and don't have any other data to add, you can 'Skip today.' This means that you're just actively telling the app that you don't have any data to add. You can at any time during the day open up your app again and add new data, such as period, LH results or sex.
'Skip today' can only be pressed for Today, and not for previous days.
This is also shown in the Calendar view, with the small arrow indicating the days that you have skipped, along with the little checkmarks that show which days you've added data.