What is a plugin?
Gatsby’s plugin layer includes a wide variety of common website functionality that you can drop in to your website. There are many types of Gatsby plugins, including:
Integrations, or “source plugins”. These plugins pull data into Gatsby’s GraphQL layer and make it available to query from your React components. Gatsby has source plugins for a wide range of headless CMSs, databases and spreadsheets, as well as the local filesystem. Here is a guide on sourcing data.
Performance enhancements while using CSS libraries, like Sass, styled-components and emotion. These plugins are not required to use these libraries but do make it easier and faster for the browser to parse styles.