On the plus side, SiteGround offers free automatic daily backups, access to the Cloudflare CDN, high-performance SSDs for all plans, unlimited email accounts and integration of the free LetsEncrypt SSL certificate into sites. The company does limit bandwidth and storage, but even those who claim to offer so-called unlimited bandwidth and storage really have some limits in its terms of service.
A Content Distribution Network (CDN) is a network of servers distributed geographically. The purpose of a CDN is to deliver content on the internet much faster. If a user in Toronto is trying to access your website, the closest server to them will take care of this. But someone coming to your website from Sydney, Australia, will be catered to by a server closest to them. The main host server holds your website content, and all the other servers have a cached version each.