I don't know that hotel at all, but I'd guess that "junior" means "small". And small in a pricey city like Barcelona can be very small indeed. But that is just a guess. I know I recently saw square footage shown in tiny print for various types of rooms on some website. It might have been booking.com. Why don't you take a look there and see if you find any clarification. I tried, but both of my randomly-selected dates are sold out on booking.com, so I couldn't see the details about the rooms.
While you're on the booking website, read the reviews to be sure there aren't any negatives that would be significant for you. I'd think noise might be an issue on some streets in the Barri Gotic, what with the restaurants and bars in the area. Keep in mind that the Spanish dinner hour begins at 10 PM, so perfectly ordinary folks will be happily walking down the street, conversing, at 1 AM or later. I didn't usually have an exterior-noise problem in Spain because my tiny single rooms often looked out on airshafts. I don't know whether there's any risk of that in a double room. I see that the hotel has a rating of 8.2 on booking.com, which is a very healthy rating.
You might also simply email the hotel and ask about the distinction between junior rooms and regular rooms.
