It's a parliamentary tool which is the opposite of roll call vote, basically where they can avoid recording important stuff like the way a representative votes.
The chair of the committee / speaker of the house / president of the senate generally says "Ok guys, we're probably doing some underhanded and sneaky shit here, and naturally we don't want to be held accountable, so we're going to have a voice vote... How vote you?" A bunch of people shout Aye or Nay, at which point the speaker / chair / president pretty much arbitrarily decides whether or not he heard more Ayes than Nays, generally says "sounds like they Aye's have it.", and unless someone on the committee requests a roll call vote, which then may or may not be granted, we're pretty much stuck with whatever asinine bills our so called representatives decide to choke down the gullet of Democracy. [fail]

