What does LExIM stand for?
LExIM is the new overall release and extension strategy:

LEAP BY EXTENSION. ITERATE BY MONTH.


  It basically means that:
  
  
  CiviCRM development will now happen mostly through extensions, enabling the users to individually turn these new features on or off by enabling these new extensions
  Releases are going to be published on a predictable monthly schedule, facilitating planning and execution of regular updates by CiviCRM service providers (from here)
  


See also: blog post and wiki article.