A Wildcard is something that matches anything. When speaking of DNS, a wildcard record is one that will be returned for any matching query. For example, a wildcard record can be defined or published as *.example.com, and any request that matches will receive that value. So apple.example.com or zebra.example.com will match this wildcard, and so will _dmarc.example.com or example.net._report._dmarc.example.com.