The plaintext is unencrypted and unsigned text. It's the payload which then get's signed and/or encrypted.

Wikipedia defines plaintext as:

In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is often used as a synonym. Before the computer era, plaintext most commonly meant message text in the language of the communicating parties. Plaintext has reference to the operation of cryptographic algorithms, usually encryption algorithms, and is the input upon which they operate. Cleartext, by contrast, refers to data that is transmitted or stored unencrypted (that is, 'in the clear').