What's the meaning of '@' in a DNS zone file?
Solution 1:
RFC 1035 defines the format of a DNS zone file.
... on page 35 you'll find:
@ A free standing @ is used to denote the current origin.
This means that @ is a shortcut for the name defined with $ORIGIN.
You can find more information on $ORIGIN here, which is an excerpt from Pro DNS and BIND, published by Apress.
Solution 2:
It's the root, or in your example it's mydomain.com.