Microdata
Microdata is a method of marking up web page content semantically, in which structured information is anchored directly in the source code using HTML attributes such as itemscope, itemtype and itemprop.
In practice
Google now recommends JSON-LD as the preferred format for structured data, because it can be maintained independently of the visible HTML and leads to errors less often. Microdata remains valid and is still processed by search engines, but in practice a move to JSON-LD is almost always worthwhile, particularly during relaunches or on new projects. If you inherit existing microdata markup, migrate it step by step rather than maintaining both formats in parallel.