Hudson is a tool for offloading the deployment and automated testing of a software application. You write your code, you push it into version control, and Hudson takes over the tasks of grabbing that code, running an installation/deployment process, testing the application (if you provide it with tests), and reporting back to you those test results.
Steve Merril of TreeHouse Agency at Boston Meetup
Hudson Continuous Integration
Once you know all your tests pass, you have everything you've written simpletests for covered.