An Environment is a grouping of pipelines and agents. By assigning an agent to an environment, it will be used to run only those jobs that belong to the pipelines of that environment. The Environment variables are used to set the environment that is required for the jobs to run.
The 'Environments' page displays all environments, every pipeline and material associated with each environment, information on what is running in each environment, an icon informing the user if there are new revisions, a button which allows to deploy the latest revisions for each pipeline, a pipeline material breakdown, and a graphical status bar for each pipleline. The detailed view of environment shows the pipelines, agents and environmet variables set for that environment.
More information about this feature is available here.