General :: Logrotate - Purpose Of Rotate And Postrotate?

I have been trying out in learning with logrotate command and logrotate configuration file )logrotate.conf custom logfile for an process is 'test.log'

Code:

#cat /etc/logrotate.d/test
/var/log/test.log {
rotate 4

[code]....

whenever the log file (test.log) exceeds 100M a new file will be created with the file name as test.'date'.'gz'(new file is created with a current date and in a compressed format of gz) and also with permission mentioned above). I really dont know what is the role of rotate( will this be carried on only for next 4times i mean upto 400MB; (4times*file reaches 100MB)? and also what could be the purpose of postrotate?