It does exactly what it says in the box: turns your TypeScript `types` / `interface` into machine-readable JSON schemas.
The library has a few open issues (does not deal well with some edge cases of composing Omit<> on sum types, and does not support dynamic (const) keys), but compared to manually writing JSON schemas, it's been amazing!
EDIT: I should add that the library supports adding further type constraints that are supported by JSON Schema but not by TS by using JSDoc (for instance, pattern matching on strings, ranges on numbers, etc.).