The bill would, according to this webiste "return habeas corpus rights to terror suspects imprisoned at Guantanamo Bay military prison." It would allow the detainees the right to challenge their detention in US Courts since that right had been revoked by the Military Commissions Act of 2006.
Why is Habeas Corpus so important? Law professor Jonathan Turley explained on last night's Olbermann (transcript from mcjoan at dailykos)
And when those placid Canadians start protesting, you've got a big problem, eh?First of all, habeas corpus is sometimes treated like some trick by a Philadelphia lawyer. It is actually the foundation for all other rights. When the government throws you into a dungeon for what you say or who you pray to, it's habeas corpus that's the right that allows you to see the enforcement of the other rights. So without habeas corpus, the rest of it is just aspirational and meaningless.
The danger when you walk away from these values, these rights that define us have been proven by this president. The greatest irony of the Bush Administration is that his legacy will be to show the dangers of walking away from those rights that define us. We’re very much alone today. He can’t go to Canada without people protesting, Miss America can’t even go to Mexico without being booed. We’re viewed as a rogue nation and it is a dangerous world to live in when you’re alone.