Application Guard is an enterprise security feature that isolates untrusted documents in an exceedingly virtualized sandbox to guard your users against malicious and potentially harmful threats. When users encounter documents from untrusted sources that are not malicious, they will still be productive without fear about putting devices, data, or identities in their organization in danger.
To help protect your users against zero-day exploits and other advanced attacks through Office 365 documents, Microsoft announced the general availability of Application Guard for Office.