This was a sacred stone that the High Priest wore on his chest plate. He used it to determine God's will. (See: [[rc://en/ta/man/jit/translate-unknown]])
This means that Joshua will have authority to command the movements of the community of Israel. "Going out" and "coming in" are two opposite commands used to emphasize that he will have full command over their movements. Alternate translation: "He will command the community's movements" (See: [[rc://en/ta/man/jit/figs-merism]])
Both of these phrases mean the same thing, and they are used together for emphasis. Alternate translation: "both he and the entire nation of Israel" (See: [[rc://en/ta/man/jit/figs-parallelism]])