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.  

Always!

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.