Giriraj,
PMI = Perfomance Monitoring Infrastructure
Basically, it is a bunch of data collectors that you can turn on to see what is going on in your WAS enviroment.
More information about this feature you can find in
WebSphere 6.1 Infocenter, find for PMI
There is also an API, but it is not well documented, search for it in the link above.
Regards,