From ACF page:
Local JSON is a new feature added in version 5 which saves field group and field settings as .json files within your theme. The idea is similar to caching, and both dramatically speeds up ACF and allows for version control over your field settings!
This plugin require Advanced Custom Fields or Advanced Custom Fields Pro plugin to work.
Installation:
/wp-content/plugins/plugin-name
directory, or install the plugin through the WordPress plugins screen directly.