"One good turn deserves another" is a colloquial expression that means "If one person helps a second person, the second person should help the first person one time when needed." I used "Help" but it could be in any context, and not just help. Other colloquial expressions that mean something similar include: You scratch my back and I'll scratch yours. What comes around goes around. (Similar, but not exactly the same thing. This one refers to karma of choice rather than deciding to help). In context, your sentence means "I'll help other people so long as they understand I expect them to help me, too." HTH. :)

善有善报