The Main purpose of Reporter.ReportEvent in QTP is it is a
Reporting Machanism in Qtp.
It means Reports an event to the test results. 

Syntax 
Reporter.ReportEvent EventStatus, ReportStepName, Details 
[, Reporter] 

EventStatus:-Status of the report step: 
0 for MicPass
1 for MicFail
2 for MicDone
3 for Mic Warning

ReporterStepName:-Name of the intended step in the report 
(object name). 

Details:-Description of the report event. The string will 
be displayed in the step details frame in the report. 

Example 
The following examples use the ReportEvent method to report 
a failed step. 

Reporter.ReportEvent 1, "Custom Step", "The user-defined 
step failed." 

or 

Reporter.ReportEvent micFail, "Custom Step", "The user-
defined step failed."