"Wolno" is an adverb derived from the adjective "wolny".
In another context it may be a defective verb (a verb that has very little conjugation, see here meaning "it is allowed", like: "Tu wolno palić" = "One can smoke here".
As for your question, I think you asked about the adverb... but if not, please precise your question.
In the comment section of other sentences it has been said that the adverb should avoid being at the end (eg dobrze wychowujemy nasze dzieci as opposed to wychowujemy nasze dzieci dobrze). How come it is not the case for this question and wolno is allowed to be at the end? Is it because it is a question? Would a statement have to be 'wolno piszecie' rather than 'piszecie wolno'? Thank you.