When do I carry out unit testing in the development workflow?

I teach myself to code. I've read about TDD/BDD which encourages programmers to write a failing test first, and subsequently write a code to make the test pass.

But then I encounter the continuous integrating (CI) workflow - after the codes are committed, and test passes, it will be deployed to the production immediately. How does the test in the CI workflow differs than the one developers write?