An Environment groups test runs within a test plan. Example values for environments include "Test environment/test system" and "Live environment/live system".
The global project settings allow you to define an unlimited number of environments. For each test plan, you can have as many test runs per test plan as necessary.
An environment has a single cumulated test result (traffic light colors) that is calculated from its child elements (test runs). This gives you a very quick overview of the overall status of a single test environment.
Please note that the cumulated test result of the most recent test run finished counts as the test result of the environment.