It is a nice room. The question “it’s a nice room, isn’t it?” Is a type of question tag. It is formed by a statement, and then negating that statement (if the statement is a negative, then the tag will be affirmative). The answer to the question here is “yes”, then it goes on to explain more by saying “it is”. This strengthens the “yes”. To learn more about question tags, see this link: http://www.grammar.cl/Intermediate/Question_Tags.htm

"isn't it" is a question tag. Question tags are short questions used at the end of a sentence. We form a question tag with an auxiliary verb (e.g. be, have, can, do) and a pronoun (e.g. he, she, we) We usually use negative qt after affirmative sentences(and vice versa). And we use them to confirm what we said, so they can be translated like "right?". so in this case, yes, it is a nice room.

Like the others have mentioned above, it is a nice room. Another way to phrase this sentence could be: It's a nice room, don't you think? Or: It's a nice room, right?