I am not a native german speaker, but this seems to me like an example of compounded words. Mit = with, Arbeiter = employee. "with employee" or "co-employee" or "colleague".
I was very happy that i figured out this meaning without hints by just breaking up the word! This is one thing i absolutely love about German: sometimes the grammar is annoyingly hard, but the wonderful vocab with so many compounded words, that you can figure out simply by breaking them down, makes it a lovely language :D
Thanks, I'm targeting a 365 day streak, so I really want to get there and learn something at the same time :) I'm pretty much addicted to German myself, but I mainly have to thank Duolingo for that! It could so easily have been another language like Spanish, but I'm really glad I picked German in the end!