SigningAKey is an OpenPGPConcept that denotes trust in the KeyValidity of the key being signed. By signing a key, you are indicating that you trust that it belongs to it's owner and that they are who they say they are.
It is important that you only sign keys where you have personally verified the owners identity, and their possession of the key. You should exercise much care in signing keys, as others may want to use your signatures to determine CalculatedTrust for a key they cannot personally verify. See KeyValidity for instructions on how to verify the validity of a key.
See KeySigningScripts, especially the section on 'caff'.
The OpenPGP standard defines several different types of signatures that can be used to denote varying levels of trust in the identity of the key's owner. These are
|0x10||Generic certification||The issuer of this certification does not make any particular assertion as to how well the certifier has checked that the owner of the key is in fact the person described by the user ID. Note that all PGP "key signatures" are this type of certification.|
|0x11||Persona certification of a User ID and Public Key packet||The issuer of this certification has not done any verification of the claim that the owner of this key is the user ID specified.|
|0x12||Casual certification of a User ID and Public Key packet||The issuer of this certification has done some casual verification of the claim of identity.|
|0x13||Positive certification of a User ID and Public Key packet||The issuer of this certification has done substantial verification of the claim of identity.|