It is the right to be brought into court.
It is fundamental to - and a sort of shorthand for - the right to be in a legal system, with laws and judges, evidence and a defense.
"Hi, there John Doe," says the policeman at the door. "We've come to take you away."
"But I'm Jane Roe, not John Doe," you say, which is true, and you have the body parts that support such a distinction, as well as some paperwork. "And why are you taking me away?"
"Don't really care who you claim to be, and the charge is none of your business," says the more talkative of the two officers, or soldiers, or whoever is grabbing you.
"Wait, let me tell my family and call my lawyer," you say.
"Not a chance," says the friendly police person, cuffing you and throwing a bag over your head.
"Blouff, blouff, blouff," you cry through the hood.
"That'll teach you to sneer at the president," says the talkative officer, kicking you to make you move. "And undermine his War on Terror!"
"It's not against the law to sneer," you try to say, but they can't hear you. And neither will anyone else, because you have no right of Habeas Corpus.
Under Habeas Corpus, you have the right to say, I want to be brought into the court to determine if I am the right person charged, if there's an actual law prohibiting what I'm charged with, if the people who are holding me have the jurisdiction to do so, and I want that publicly known and I want the right to dispute all of that and the right to be tried too.
Without Habeas Corpus you can be swept up off the street and never heard from again. Period. Nobody has to know. Nobody including yourself has to know why. Nobody gets to determine if there is a law against what you're charged with. You have no rights at all.
In America, the Constitution forbids taking habeas corpus away from you (except "in case of rebellion or invasion the public safety shall require it.") It was written that way because the right of habeas corpus was a basic right even under the King of England. It was the most basic check on a king's tyranny. It was assumed.
Attorney General Alberto Gonzales now asserts something he thinks is very tricky and clever. What if you never had such a right? Then not giving you the right of Habeas Corpus would not be taking it away. That would allow his president to order someone snatched up off the street, or from their homes, and then anything could be done to them. Prison for life. Tortured. Killed. Sent to another country. And nobody would even have the right to ask if our government imprisoned that person. Or why. Or if it was the right person.
Gonzales' assertion is intended to do two things.
First, it is to allow the government to remove rights that are so fundamental that they are pre-constitutional, that they enable all our other rights.
The second, is a defense in advance for the president and his minions. They can claim that they thought what they were doing was legal. After all, the Attorney General of the United States wrote them a memo that said it was. And if, he was wrong, well then, if it goes to court and the court says so, then they'll stop. See, they acted in good faith.
Habeas Corpus means you are in a society of laws. Without it, you are in the land of Saddam Hussein, August Pinochet, Adolf Hitler, Josef Stalin, Ivan the Terrible ... and ...