Currently, this notation changed to "Last 30 Days".
Most of the data is only available for the most recent 31 days (except page views and session starts). The previous month could be outside of the range of the last 31 days. Therefore you will not be able to see e.g. bounce rates and in Analytics.
Tip: You can always set the period for the last 31 days by entering the dates in the custom fields and clicking "Apply period".