The Red-Green-Refactor process is the most important part of TDD. It is the main pillar, without which no other aspect of TDD will work.
The name comes from the states our code is in within the cycle. When in the red state, code does not work; when in the green state, everything is working as expected, but not necessarily in the best possible way. Refactor is the phase when we know that features are well covered with tests and thus gives us the confidence to change it and make it better.