... But I'm not sure what it is.

Since Italy like most of Europe drives on the right, it is not uncommon to see signs pointing in the other direction indicating that you must turn right. I haven't found any reference which indicates what this sign is meant to represent, but is probably safe to assume that you must turn left. And you must do so, apparently, between 10 AM and 7 PM during festivals and between 9 PM and 1 AM every night.

I can only assume that for some reason traffic needs to be kept out of the area to the right during those times.