Environment is a group of robots, so you can allow only certain robots to have access to run certain processes. Execution target is the robot on which you want your process to run, so you can select the robots from the list that appears at the bottom of the job “Start Job” screen. If you just want your process to be run on any robot within an environment, you can choose “Allocate dynamically” when starting a job and it will run the process on the first available robot within that environment.
This also may provide some additional information: Environments
I hope that helps!