Schema, or schematic markup, is additional HTML (JSON) that is applied to any variety of webpage items to further help Google's search engine to better understand and display it in search results pages (SERP).
You may have noticed more visual components with search results, these are in part from the use of schema.
Example of an instant answer:
Example knowledge card:
Schema isn't just about creating visuals in SERP, it is also helping fuel adoption and advancement of voice search. At its core, schema is about advancing the categorical identification of web components so as to be better comprehended by partnering systems.
Here are a few useful tools and resources to help with schema.