What is task response in IELTS?

Task response (Task achievement in Task 1) measures whether you actually answer the question set: addressing every part of the prompt, taking a clear position, and developing your ideas with relevant support. You can write fluent, error-free English and still score poorly if you drift off the question or only half-answer it.

It's the criterion students most often lose marks on, usually by misreading the prompt or running out of time before developing their points. Planning for two minutes fixes most of it.