Environment is a grouping of Robots, that is used to deploy processes. The Environments page displays all previously created environments and enables you to manage Robots within them. If you have multiple Robots on the same machine (such as Robots in User-Mode or Robots as a Windows service), it is recommended that you group them in the same environment. Otherwise, some errors might occur when deploying different versions of the same process.
For more info refer to https://www.edureka.co/community/43016/what-an-environment-uipath-orchestrator-and-how-create-one