You can use this to exclude a certain time period (for example, only consider data from the previous month without including this past week), or to set a 30-day period that begins on a determined day.
For example, say you want to grab data for a whole month's time, but for the past 5 days, you've had a particular promotion or offer. Including these past 5 days could mean the overall numbers might spike in an uneven direction, so you need to exclude the past 5 days from the analysis.
When using a custom date range, it's important to keep in mind that the system considers number of days and not specific dates.
The way to do this would be, as seen on the screenshot below, set a Start Date for 35 days ago, and the End Date for 6 days ago (5 previous days to exclude and today). This way you'll exclude from the past 5 days from the recipe, and you'll still get data for a whole month's time.
The use of custom date ranges should go hand in hand with the frequency you are using to schedule a recipe.
You can add multiple Custom Date Ranges, and once created, you'll see them on the Date Range drop-down within the conditions settings.