Answer: “One” means that there is one form which governs and connects everyone together, unites and compliments all based on the infinite number of internal properties that are opposite one another in all possible senses.
Such a connection is called “One.” It points at the infinite number of the individual parts included in Him, their form of connection and everyone’s uniqueness in Him. All of these are included in the notion of One.
One includes the entire reality: When He was one whole, but then divided into many parts, and went through all corrections. Everything that has been attained during these corrections, all of these, is called One. This means that He includes absolutely everyone within Himself. He is not simply One, but rather He is One since everything is included within Him.
From the 2nd part of the Daily Kabbalah Lesson 10/21/12, The Zohar