With Development Mode, features that do not require a connection to WordPress.com servers can be activated on a localhost WordPress installation for testing.
You can turn on Development Mode by adding
define( 'JETPACK_DEV_DEBUG', true); to your wp-config.php file. Make sure you add it before the
/* That's all, stop editing! Happy blogging. */ line.
Development mode automatically gets enabled if you don’t have a period in your site’s hostname, i.e. localhost. If you use a different URL, such as mycooltestsite.local or something, then you will need to define the JETPACK_DEV_DEBUG constant.
You can also enable Jetpack’s development mode through a plugin, thanks to the
add_filter( 'jetpack_development_mode', '__return_true' );
While in Development Mode, some features will not be available at all as they require a WordPress.com for all functionality—Related Posts, for example. Other features will have reduced functionality to give developers a good-faith representation of the feature. For example, Tiled Galleries requires the WordPress.com Photon CDN; however, in Development Mode, Jetpack provides a fallback so developers can have a similar experience during development and testing.