JetPack is a collection of WordPress power tools maintained by the WordPress creators Automattic. It will, among other things, provide you with site stats and analytics, automatic social network sharing, 24/7 uptime monitoring, and access to a high-speed content deliver network for images.
Many of JetPack’s features use the WordPress.com infrastructure, and to use it on a self-hosted WordPress install – like the one you’re looking at right now – the WordPress XML RPC interface has to be accessible to the WordPress.com servers. The problem with that approach is that XML RPC interface is one of the favorite attack vectors for WordPress
hackers script kiddies. So the interface is ideally locked down and made inaccessible unless it’s strictly necessary to make it available.
To get JetPack to work properly it’s necessary to make the XML RPC interface accessible from the in-ter-net. But you don’t want every single Russian basement dweller to get access: Ideally, you just white list the JetPack servers.