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.
Unsure which CI environment is being used?
Check for the configuration file. .travis.yml is Travis, circle.yml is Circle CI.