Can someone clarify what this Joel On Software quote means?

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Published on 2010-03-15T20:57:08Z Indexed on 2010/03/15 20:59 UTC
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I was reading Joel On Software today and ran across this quote:

Without understanding functional programming, you can't invent MapReduce, the algorithm that makes Google so massively scalable. The terms Map and Reduce come from Lisp and functional programming. MapReduce is, in retrospect, obvious to anyone who remembers from their 6.001-equivalent programming class that purely functional programs have no side effects and are thus trivially parallelizable.

What does he mean when he says functional programs have no side effects? And how does this make parallelizing trivial?

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