Guide: Test-Driven Development (TDD) for PMI-ACP Exam

Test-Driven Development (TDD) is a software development approach where the developer writes a test case that defines a desired improvement or function, then produces the code to pass that test, and finally refactors the new code to acceptable standards.

Why it is important:
TDD is important because it ensures that the code functions as expected, reduces bugs, and improves the software design. It also enhances the understanding of the requirements and promotes the building of simple designs and solutions.

How it works:
TDD works in a repeating three-step process.
1. Write a test: The developer first writes a test for the smallest possible functionality to be added.
2. Run all tests and observe if the new test fails: This validates that the test harness is working correctly.
3. Write code to pass the test: The minimum possible code is written to pass the test.
Finally, the developer refactors the code to improve the software structure without changing its behavior.

Exam Tips: Answering Questions on Test-Driven Development (TDD)
When answering questions related to TDD in the PMI-ACP exam, always remember that:
1. TDD is about writing tests before writing production code.
2. It's a 3-step process: write a test, watch it fail, write code to pass the test.
3. Refactoring is an integral part of TDD.
4. TDD ensures the code is working before it’s written, so it helps reduce bugs, clarify requirement and simplify design.

Thus, understanding TDD to its very core and applying it correctly in practice is key to answer any question related to it in PMI-ACP exam.