What is TDD and what is it used for?

Question

Test Driven Development

TDD is when you first write the test and only then the code, and not vice versa?
That is, first the text (spec) is written, then the code that will pass this test. If you need to expand the functionality, then again, first the test is expanded and the code is corrected for it? Am I understanding everything correctly or is there something else?
Or do I not understand everything correctly?

Answer 1

TDD is when you first write the test and only then the code, and not vice versa?

As if yes, but rather than just writing the test itself, but writing the client code (after all, what are you writing the test for? for the code that will be coming soon), that is, you are already designing your program at this stage - what will work and how, and tests show how this code should behave in different situations,
then you will implement the already designed code

Answer 2