A closed row is a row that is no longer in effect in the database. It is a row that has either been (logically) deleted from the database, or otherwise superseded by a row modification, which closes the original row, and opens a new row with the changed information.
In a transaction-time or bitemporal table, a closed row is a row with a transaction-time period that has an ending bound different from UNTIL_CLOSED (9999-12-31 23:59:59.999999+00:00). Such a row is said to be closed in transaction time.
The concept of a closed row does not apply to valid-time tables. See No Longer Valid Row.