What is the meaning of Plain_text?

Text or any data that is to be encrypted (as opposed to ciphertext).

Human readable text which consists only of a string of characters, represented using a character encoding such as ASCII or Unicode. In the file format sense, plain text may represent structured data in a human readable format such as XML.

Data which consists only of human-readable unformatted text, as opposed to machine-readable binary data or formatted/structured text. In this sense, the character data in between XML tags may be called "plain text".

Alternative spelling of plaintext

Source: wiktionary.org