Definition
Plain text is data stored as a sequence of human-readable characters, without proprietary encoding or hidden binary structure. The principle of keeping knowledge in plain text says that source code, configuration, documentation, and data should favor formats a person can read and a simple tool can process.
This stands in contrast to opaque binary formats, which lock information inside a specific application and become unreadable when that application disappears or changes.
Why it matters
How it works
The strength of plain text comes from leverage. Because the format is universal, every text-processing tool ever written becomes available: editors, grep, version control, build pipelines, and scripting languages all speak it natively. Diffing two text files shows exactly what changed; diffing two binary blobs usually shows nothing useful.
Plain text is not always optimal. It can be larger and slower to parse than a tuned binary format. The pragmatic stance is to default to text and choose binary only when measured performance needs demand it.