Question
What does the @ symbol represent when used inside of a DNS zone?
Answer
When editing a DNS zone, you can use the @ symbol to reference the base domain of the zone.
For example, if you are editing the zone for example.tld, you could write an A record like this:
@ IN A 192.0.2.2 ; IPv4 address for example.com
That would be equivalent to:
example.com IN A 192.0.2.2 ; IPv4 address for example.com
This is outlined in the following RFC on page 34
RFC 1035 - Domain Implementation and Specification