About the usage of +function ($) {...}(jQuery);

About the usage of +function ($) {...}(jQuery);

Hi:
I recently read the codes in bootstrap, and see the codes snippet as following:
  1. +function ($) {...}(jQuery);
the main problem is I didn't see +function before, and also can not find out any 
explain about this usage, what is that means, and when to use it.
Is there any material about that? Thanks a lot.