What it is Habeas Corpus?

It was a very quaint idea from our distant past.

It means they have to tell you why you are locked up and it is supposed to be a legal reason.

Habeas Corpus is a writ wherein you claim that the government has a person without legal authority to have him locked up.

In practice, it is a means of getting someone out of jail that has been denied due process. Writs of Habeas Corpus are filed against the warden of a prison saying that he is being held illegally. It is basically an end-run around the whole appellate process. Often, an appeal is not possible because the time for it has expired, etc....A writ of habeas can be used to handle those situations. It has a lot of limitations, however.

Originally, the writ (often called the great writ) was designed to get someone out of jail who was locked up and just forgotten about. The process in the US is pretty consistent now, so that doesn't happen often.

"Definition: Habeas Corpus, literally in Latin "you have the body" is a term that represents an important right granted to individuals in America. Basically, a writ of habeas corpus is a judicial mandate requiring that a prisoner be brought before the court to determine whether the government has the right to continue detaining them. The individual being held or their representative can petition the court for such a writ.

According to Article One of the Constitution, the right to a writ of habeas corpus can only be suspended "in cases of rebellion or invasion the public safety." Habeas corpus was suspended during the Civil War and Reconstruction, in parts of South Carolina during the fight against the Ku Klux Klan, and during the War on Terror."

You can't hold a certain person without telling them why.

A writ of habeas corpus ad subjiciendum, also known as "The Great Writ", is a summons with the force of a court order addressed to the custodian (such as a prison official) demanding that a prisoner be brought before the court, together with proof of authority, allowing the court to determine whether that custodian has lawful authority to hold that person; if not, the person shall be released from custody. The prisoner, or another person on his behalf (for example, where the prisoner is being held incommunicado), may petition the court or an individual judge for a writ of habeas corpus.

Basically they can't arrest you, and hold you in prison without you knowing a reason why.

It is far from "quaint"; it is one of the most important concepts in our criminal justice system. Without it, the police don't have to prove to the court that they have a good reason for keeping you locked up, and you can just rot until they decide they want to try you (or not).

Wikipedia has a good writeup on it that will give you the information you seek; the link is below.

It means "produce the body" literally

It means you have a right to come before a judge to hear what you have been charged with. It is guaranteed in the US constitution to all Americans- yet Bush took it away from Jose Padilla and threw him in jail not allowing him to be charged or tried until the SC told him he couldnt