The Node.js assert module is a vital tool for testing expressions and ensuring program correctness by verifying that certain conditions, known as invariants, hold true during execution. If an invariant's test returns false, assert throws an error and halts the program, which is useful for debugging and maintaining code integrity. The module offers various methods, such as assert.ok(), assert.deepStrictEqual(), and assert.fail(), each serving specific testing purposes, like verifying strict equality or adding custom error messages. Using assert in strict mode is recommended for production because it relies on strict equality checks, avoiding the pitfalls of abstract equality comparisons in legacy mode. The guide provides practical examples of implementing these methods to help developers effectively identify and trace errors in their Node.js applications.