All our ratios - except the 5Y versions - are based on the trailing twelve months data. This is a representation of the financial performance for the 12 months before a certain end date. The Period end date displays the end date of this period.
This column shows how 'fresh' the data is and can be used to filter out stale data. A company could for instance become delisted and as a result it will no longer report financial performance. By setting a column filter to exclude companies with a period end date before a certain date, you ensure that these companies are not included in your list. Another way to do this is to use the 'Results Age' in the filter menu, which comes with preset periods. (6 months, 9 months, etc...)