No modern framework would be complete without the ability to test the application code and functionality. FuelPHP has been built with this in mind and, thus, includes tests and test cases based on the PHPUnit testing framework.
Unit tests are automated tests written to check whether or not the units of functionality (methods and functions) are working as expected. The tests typically test whether or not, for a given input, the output is correct, treating the functions as a black box to ensure that the internal logic works.
Since unit testing is automated, it's easy to ensure that the recent code changes don't break other functionality. It also allows for the use of a continuous integration server such as Jenkins (http://jenkins-ci.org). A continuous integration server will automatically deploy your code for you once the code passes the unit testing, allowing you to concentrate on the actual code.