When is Unit Testing done in the software development process?

I am aware of what Unit testing is but When I read about Test Driven Development, I got confused.

Is unit testing something you do ahead of the code-base following the test first then develop process? or is it something you do mid or after development so you would just refactor some parts of the code-base(which is what I am doing in the project).

I would highly appreciate any enlightenment.