In cryptography, plaintext is the information which the sender wants to transmit to the receiver(s).
Before computers, plaintext meant text in the language of the communicating parties. Since computers, the definition has been expanded. It now includes not only the electronic representation of text, such as emails and word processor documents, but also the computer representation of speech, music, pictures, videos, ATM and credit card transactions, sensor data, and so forth. That is basically any information which the communicating parties might wish to hide from others. The plaintext is the normal representation of the data before any action has been taken to hide it.
The plaintext is used as input to an encryption algorithm; the output is termed ciphertext. Some systems, however, use a cascade of layers of encryption (called 'rounds') in which case the ciphertext output of one encryption algorithm becomes the plaintext input to the next. Triple DES is an example.