Test driven development is where developers write all of their tests before they write a single line of code. For example, if their code needs to produce a specific message, then they can write a test that creates those conditions which then can be checked to see if it will run, and that the message they are expecting does appear. The advantage of this is that you can clearly see the tests that will be run on the code, it can be peer reviewed, and you can more easily see that no section of the code is being missed. That way you can be sure that the end result is of the highest quality.
Find out more about TDD here.