A Domain Name System (DNS) zone file is a text file that describes a DNS zone. A DNS zone is a subset, often a single domain, of the hierarchical domain name structure of the DNS. The zone file contains mappings between domain names and IP addresses and other resources, organized in the form of text representations of resource records.
Most importantly in terms of DMARC, a zone file contains all current DKIM and SPF records - information that is critical in ensuring the move towards full protection is a clean and seamless one.
Zone files can generally be requested from your domain provider, via their support channels. It can come in the form of a .txt file, or even in the contents of an email.