1. The difference in meaning: "just go" can either mean "leave here" or it can mean "go somewhere in particular." But "just go away" only means "leave here, I don't care where you go." 2. Difference in tone: "just go" can be a helpful or encouraging command/suggestion (example: "Don't be nervous. Just go and have fun.") but "just go away" almost always has an angry/impatient tone, and is used when the speaker is angry or annoyed at the other person and just wants that person to leave. 3. Here, "just" means "don't delay or do anything else, but instead immediately go."

When you say "just go" or "just go away" you give an order with no explanation in both case, but in the first case you can add a destination, in the second one you can't. The slight difference is that "go away" means to go far, the other person understands that you don't want to see him/her. When you use "just go" you don't especify the destination, that's why you can add the destination. Now, in this case, the word "just" orders doing nothing but the action indicated. Another uses could be: Just write the sentences Just clean your room Just Turn on the lights I hope it helps, if you have more doubts contact me:D