A simple explanation as it was given to me...
The kanji for "ai" represents a pot with a lid on it. Hence, "ai" symbolizes to two things fitting togeter. Not to be confused with "wa" which refers to harmony.
The kanji for "ki" represents a boiling pot filled with rice. Hence, "ki" symbolizes energy.
So, I would have to say it means to fit, join or combine energy.
Its no "circle of life" explanation, but it makes sense to me.