Data Guard :: Standby Redo Log On Standby

GROUP# THREAD# SEQUENCE# sizeMB ARC STATUS T
---------- ---------- ---------- ---------- --- ---------------- -
1 1 115 512 YES CLEARING O
2 1 116 512 YES CURRENT O
6 1 114 512 YES CLEARING O
4 1 112 512 YES CLEARING O
5 1 113 512 YES CLEARING O
3 1 111 512 YES CLEARING O

6 rows selected.
[code]...

Above result is from standby database.I am trying to understand the concept of standby redo log.My stadby database protection_mode is MAXIMUM PERFORMANCE.