free Wifi tethering/hotspot ?¿?

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http://androidforums.com – Hey... So.. i know this gonna sound nooby and stupid but.. I've been thinking about rooting and searching why would most people wanna root their android phones and it turned out to be mainly for getting free wifi tethering or the free hotspot the quistion is... what exactly is this free wifi tethering i really want to know everything about it. i hope someone here explain it to me >_< thanks in advance :) (General)