Until today the generation of review and product rich snippets in Google search results required either microdata or RDFa. Unlike these two inline structured data markup syntaxes, JSON-LD can be placed anywhere in an HTML document with a
<script> tag, entirely separate from the page's presentation layer.
Today's change for reviews and products (which employ the schema.org types Review, AggregateRating and Product) is the latest of Google's incremental efforts to support the protocol, following on the heels of JSON-LD support for event rich snippets and Knowledge Graph features early in 2015, and recipe rich snippets in July 2015.
The Google Developers pages for product and review rich snippets have been updated to reflect the fact they're now supported by JSON-LD, but other documentation is still in the process of being changed.
Mueller also said in his post that "we've cleaned up some of our application logic. For example, requirements for explicit reviewed item and correct property name values are now enforced." He advises verifying that a site's code is compliant by using the Structured Data Testing Tool and the Search Console Data Dashboard, and to review the structured data documentation on Google Developers.