Signature

Signing the data is basically an additional encryption. When you send the data to a partner and encrypt it, the data is first encrypted with the partner's public key (encryption). Then a hash (a kind of checksum) is formed over the data and this hash is then encrypted again (signing) with the private key of the local certificate.

The partner first decrypts the signature with the public key of your certificate, checks the checksum and, if it is correct, the data is decrypted with the private key of his local certificate.

If we only send the data signed, it is sent in plaintext, because the signing is only an encryption of the hash with the local (private) key, but no encryption of the data. The signing only proofs that we have sent the data, because only we as owner of the private key can encrypt the checksum correctly.