Any idea about this error

Target Name=XXXXX
Target Type=Database Instance
Host=XXXXXXXXXXXX
Metric=Blocking Session Count
Metric Value=2
Blocking Session ID=59.000000
Timestamp=Oct 8, 2007 10:15:01 PM EDT
Severity=Warning
Message=Session 59.000000 blocking 2 other sessions. Notification Rule Name=Blocking Lock Issues Notification Rule Owner=SYSMAN

Did anybody know what this error represents? We have been getting this error for the past 10 days between 9pm to 10.30pm and Today DBA asked us to check if there was any special processes running at that time.

As far as I know there was not special processes running at that time and the normal processes did not record any errors in the Application log. ( application log is a table where we record any database errors and application errors.) Received on Tue Oct 09 2007 - 13:19:04 CDT