public void SignPublicKey(
	long keyId,
	long signingKeyId,
	string signingPassword
)

Public Sub SignPublicKey ( _
	keyId As Long, _
	signingKeyId As Long, _
	signingPassword As String _
)

public:
void SignPublicKey(
	long long keyId, 
	long long signingKeyId, 
	String^ signingPassword
)

Signing a key certifies that you know the owner of the keys.
When you import keys into your KeyStore and are sure that the trusted third party is really the organization/person they claim to be, you can sign his/her keys.

PGP command line(r) equivalent:
pgp --sign-key keyId --signer signingKeyId --sig-type exportable

GnuPG equivalent command:
gpg --local-user signingKeyId --sign-key keyId