If someone is on your back it means that they are pestering you or consistently criticising you. When you ask someone to 'get off your back' you are asking them to stop bothering you or criticising you and to leave you alone.

1. Would you just get off my back? I'm not going to change my decision, no matter what you say!
2. That's the third time you have stopped to criticise me. Would you get off my back, please?

1. Anne was sick of her boss's constant criticisms, so she angrily told her to get off her back!
2. I'm glad Peter has finally gotten off my back about giving him a raise. He has accepted that there simply isn't room in the budget for it.