This flashback query  allows us  to query a table at a point in the past. We can retrieve which we might have deleted by  mistake in the past.

Make sure flashback is enabled:

SQL> select flashback_on from v$database;
 
FLASHBACK_ON
------------------
YES

If flashback is not enabled Refer: How to enable flashback in oracle

Delete some data:

09:10:51 SQL> select count(*) from test;

  COUNT(*)
----------
     87298


09:13:11 SQL> delete from test;

87298 rows deleted.

09:13:36 SQL> commit;

Commit complete.

09:13:39 SQL> select count(*) from test;

  COUNT(*)
----------
 0

Now use timestamp to_date to query the table in past(for particular time stamp)

09:15:53 SQL> select count(*) from test AS OF TIMESTAMP TO_DATE('29-AUG-2015 09:10:51','DD-MON-YYYY HH24:MI:SS');

  COUNT(*)
----------
     87298

Restore the deleted data

09:18:16 SQL> insert into test select * from test AS OF TIMESTAMP TO_DATE('29-AUG-2015 09:10:51','DD-MON-YYYY HH24:MI:SS');

87298 rows created.

09:19:38 SQL> commit;

Commit complete.

We can see by using flashback query, we are able get the deleted data from the past.