Its"Lein" not Lean. Lein is a rule which means a permanent govt employee whose service has been confirmed by his or her parent department can avail lein for a period maximum up to 3 years if selected on another permanent job. It's a right to come back and join parent department within 3 years. One must retain his lein before leaving his parent department and joining his new job. This will give him or her a chance to come back if he or she does not deem fit in new environment.
Hope this will enhance your knowledge about "Lein Period"!