Answer: These are quite difficult concepts.
The right line, as a rule, is bestowal, love, emanation, Light.
The left line is egoism, reception, attraction to yourself.
We must remain in two lines because a connection with man or with the Creator is built on the interaction of bestowal and reception.
The combination of both lines creates a middle line, the correct combination of properties of giving and receiving. In it, we build up a dynamic system of interconnection. For example, I receive as much as I give you, and you receive as much as you give me. By correctly adjusting our connection, we begin to feel the Creator between the two lines.
In other words, the middle line is the Creator Himself. His disclosure is built on two opposite properties, but these properties do not have anything to do with Him.
From the Kabbalah Lesson in Russian 12/10/17