Definition
This idiom is used to express that a kind or helpful act should be reciprocated, especially towards someone who has previously done something beneficial.
Since John lent me his car when I needed it, I should definitely help him move when he needs it; one good turn deserves another.
My colleagues always offer help when I need extra hands, so when they asked for help with their presentation, I did it without hesitation. One good turn deserves another.
I once assisted her in getting a promotion at work, so she was happy to support my new project. One good turn deserves another.