OK... OK... OK... It's a fancy word. Here's what it means.
Give and Take. That's it. That's all it means.
When I give you something, you give me something back.
If I give you a gift, you give me a smile and a "thank-you." If I say "Good Morning" you say "Good Morning" back.
It's that simple! And, it's amazing how many couples fail to follow this easy rule.