You can improve your REP by adding multi-sentence, informed, responses that add value to the Community you are in. By reading content and providing accurate and insightful commentary or analysis, you can grow your REP. It is not in your interest to post short, pointless, replies (they will usually get you nuked).
If you copy/past from other sources or are clearly talking out of your butt, the Admins will know and trash your REP. Best to pick 1 Community where you have good competence and focus on contributing consistently there rather than trying to be mediocre everywhere.