How this code works?

[Kuzyo]

Hi guys. <br>Have problem, how to take element from array in a random way without repeating itself. The problem was solved, but I can properly understand how it works. Here is codepen http://codepen.io/Kuzyo/pen/vdlai. <br>
Don’t understand this part:

  temp = characters[i];
   characters[i] = characters[j]; 
   characters[j] = temp;

Can somebody explain? Thanks.

[Author]

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