In the past, Apple Maps has provided reviews of points of interest from Yelp. While that's useful, Yelp reviews often don't provide the depth that is found in reviews from other sources. The Maps app has added reviews from two of those other sources -- Trip Advisor and Booking.com -- to a number of locations.
The Maps source attribution page still only shows Yelp as the source for local reviews, perhaps waiting for a wider availability of Booking.com and Trip Advisor reviews. For example, at this time my local reviews are all still Yelp-only.
The new sources aren't the only improvements Apple has made to its in-house mapping service. The company has added new locations to the 3D Flyover capability, and is adding data partners like Gas Buddy and Great Schools as well. The presence of Apple-marked vans on the streets of some major cities has raised suspicion that the company may also add a Google Street View-like feature to Apple Maps in the future.