Blork is a JavaScript library for argument checking, primarily designed for use in modules where the code's users are unknown. It provides a simple and readable way to validate function arguments and other values against a wide range of built-in types, including primitives, objects, arrays, and more. Blork also allows for custom type checkers and offers features like optional types, inverted types, and size constraints, making it a flexible and powerful tool for ensuring code correctness and robustness.