Apple does not provide much information on what's included in the iTunes 12.5.5 update. According to the company's release notes, iTunes 12.5.5 introduces "minor app and performance improvements."
iTunes 12.5.5 can be downloaded immediately from the Software Update feature in the Mac App Store. The Windows version can be downloaded using the "Check for Update" feature built into the iTunes help menu.
iTunes 12.5.5 follows iTunes 12.5.4, which was released in December and introduced support for the TV app and the Touch Bar on the new MacBook Pro models.