what's the meaning of the command: source /opt/ros/groovy/setup.bash

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Hello professors, I am so confused about the tutorial of catkin which told us that we should type the command: source /opt/ros/groovy/setup.bash every time.... I have no idea. because before I type this command, my workspace is under home directory which is /home/username/workspace, but when I type this command and then I type the 'roscd', the path turned to /opt/ros/groovy/.. so my question is where should I create my package? Does that mean every time when I want to create package I should source first? I' so confused about this.. cause when I try to create package and run it, there always be showing some error which told me that command not found....I just follow the tutorials....

Thanks so much for your instruction.