it means something like 'produce the body'
basically it means that you can't keep the person in prison if they are summoned to court
if they are summoned to court, you have to get the person to court. you can't say "oh, the prisoner is in prison so they can't go to court"
one reason habeas corpus might exist is say you got imprisoned by somebody who doesn't like you
if the habeas corpus didn't exist and the court said "hey, let's put this person on trial" then the prison commander (or whatever) could refuse and keep you in prison. forever.
so that's what habeas corpus means
it means 'produce the body'