An "on hold" question is one that has been closed for less than five days; a "closed" question is one that has been closed for five days or more. The title of the question changes, but there's no corresponding change in the state of the question.
This is explained in the Stack Exchange blog post, The War of the Closes:
"On hold" will replace "closed" on newly closed posts. The word "closed" sounded final. Think about "closed" discussions, real estate deals, or job applications. In each case,"closed" means, a) additional revisions are not welcome, and b) the matter won't be further considered.We led with a word that sounded final, so when we eventually told users they could edit their post, they weren't listening; they were dusting off the old debate uniform to argue their case."on hold" better conveys what we always meant:
If you can edit your question to better fit our model, we can get you the help you need.
Questions not re-opened within five days will revert to displaying as "closed," to serve as a clearer signpost going forward.