.prettierrc: This is a configuration file for Prettier. Prettier is a tool to help keep the formatting of your code consistent.
gatsby-browser.js: This file is where Gatsby expects to find any usage of the Gatsby browser APIs (if any). These allow customization/extension of default Gatsby settings affecting the browser.
gatsby-config.js: This is the main configuration file for a Gatsby site. This is where you can specify information about your site (metadata) like the site title and description, which Gatsby plugins you’d like to include, etc. (Check out the config docs for more detail).
gatsby-node.js: This file is where Gatsby expects to find any usage of the Gatsby Node APIs (if any). These allow customization/extension of default Gatsby settings affecting pieces of the site build process.
gatsby-ssr.js: This file is where Gatsby expects to find any usage of the Gatsby server-side rendering APIs (if any). These allow customization of default Gatsby settings affecting server-side rendering.
README.md: A text file containing useful reference information about your project.