Continuous integration, or CI, is a way that we can test code continuously. These systems will automatically listen for new Pull Requests and other events, and automatically run test suites, and other automatic processes.
We use a number of different CI systems: Travis CI and Circle CI. Normally the build status of a project is displayed as a image on a project's README file. Green means the tests have been successful, and red means they have not. Clicking the image will show you the results of these tests.