Notations provide meanings to add information to a signature or certification (which also is a special kind of signature). From RFC 4880, 126.96.36.199. Notation Data:
This subpacket describes a "notation" on the signature that the
issuer wishes to make. The notation has a name and a value, each of
which are strings of octets. There may be more than one notation in
a signature. Notations can be used for any extension the issuer of
the signature cares to make. [...]
Notations are rarely used in practice (actually I've never seen real usage, although there are few notation subpackets to be found in key server dumps). Example usage might be to add the location of an identity check during key signing, or the document types presented.
Each notation consists of a notation name and a value, while the value is an arbitrary UTF-8 encoded string.
@, this is part of the notation namespaces. Namespaces help to distinguish notations if they use the same name, but are defined by different people. Again from the same section of the RFC:
Notation names are arbitrary strings encoded in UTF-8. They reside
in two namespaces: The IETF namespace and the user namespace.
The IETF namespace is registered with IANA. These names MUST NOT
contain the "@" character (0x40). This is a tag for the user
Names in the user namespace consist of a UTF-8 string tag followed by
"@" followed by a DNS domain name. Note that the tag MUST NOT
contain an "@" character. For example, the "sample" tag used by
Example Corporation could be "email@example.com".
Names in a user space are owned and controlled by the owners of that
domain. Obviously, it's bad form to create a new name in a DNS space
that you don't own.
Since the user namespace is in the form of an email address,
implementers MAY wish to arrange for that address to reach a person
who can be consulted about the use of the named tag. Note that due
to UTF-8 encoding, not all valid user space name tags are valid email
With other words, there can be official notations in the form of
some-notation-name defined by a standard and registered with the IANA. On the other hand, everybody can declare his own notations, which should have the form
firstname.lastname@example.org. For example, if I would use
email@example.com, it would not collide with the (assumed) official notation
some-notation-name in the IANA namespace.