I'll scratch your back if you scratch mine

I was watching The Simpsons episode Season 4 Episode 17 when I heard Homer Simpson talk to Mr.Burns about a Union Deal since it seemed like Homer was President of the Union. 

Mr.Burns was trying to bribe him by giving him money so it seemed.

He said "If you scratch my back, I'll scratch yours". 

In this context it means if you help me, I will help you. It's a win-win situation. We use this phrase anytime we want equal benefit from something. If it's a kick back, or a lucrative deal it helps to use rich English phrases in expressing what you want.