TTIP and TPP: what's the deal with these "secret courts"?

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As a progressive/leftist/whatever, I get a lot of scare stories from Facebook, email and real life about the terrible ISDS courts that will destroy democracy and all things good. While I'm not taken in by a lot of the nonsense surrounding these treaties and the hyperbole that gets thrown around, some of the details that have trickled down to me sound ominous. Can someone give me the briefest run-down on why these things are needed in FTA's and if they can be replaced with something more palatable in the future?