What are the six openings on the bottom of the MacBook Pro for?
They're vents for air:



…but they're not just for air. They are also used as part of the structural system of the machine.


  The vents are also part of the structural system creating strong beams that actually increase the rigidity of the overall product.




This Apple video explains it very well:


http://www.youtube.com/watch?v=Neff9scaCCI&amp;t=3m20s