Well, literally it's Latin for "You shall have the body." Basically, it means that one cannot be illegally detained indefinitely. Like, the cops can't pick you up and leave you in jail for days or months or years without doing anything -- your lawyer would file a writ of habeus corpus and they'd have to present you to a court and either charge you with something (which means you'd go to trial in a reasonable amount of time) or let you go.