Origin Shield is an option used by many CDN providers. It allows to protect your origin from malicious attacks and also to deliver content from your origin to the certain node quicker.
Such kind of option reduces the number of requests received by your origin server. For example, when you need to purge large amount of data, Origin Shield will allow you to offload your origin - the requests will come not from all the nodes, but will be filtered by certain CDN servers, that surround and protect your server for original files. Thus, with the use of such option you will be able to protect your origin, preventing requests got by any other servers.
This way traffic will be distributed among CDN servers and the technology also allows not to use any problematic Shield server to deliver data to nodes. While using Origin Shield option numerous servers are involved, that helps to cope with any burst in traffic.