Symbols, introduced in ES6 as a new primitive data type in JavaScript, are unique and immutable values that differ from other primitives by behaving like object types in terms of identity. Initially intended to be private, symbols are not inherently private but maintain their uniqueness, which is useful for creating unique property values and keys that are not easily overwritten or conflicted with other strings. They are often employed to prevent accidental overwrites of object properties or to manage unique identifiers across different libraries. While symbols are not visible in standard JSON representations, they can be accessed using specific methods like `Object.getOwnPropertySymbols()`. However, the `Symbol.for()` method allows symbols to be stored and retrieved from a global registry, which can lead to shared values across different applications, somewhat compromising their uniqueness. Despite these limitations, symbols provide a useful mechanism for defining unique and non-conflicting keys in complex JavaScript applications.