How would it be possible to explain unity of being in simple words?

How would it be possible to explain unity of being in simple words?
Concise answer
Wahdatul Wujûd means to believe that all the entities (things present around us) are an indication of Allah's presence and to believe that everything besides Allah is subjective or just shadows and epiphany of the Truth. Conclusively there is no multiplicity in God, the Real Wujûd as the one and unique reality from which all other reality derives. This theory does not imply polytheism. That is to say, according to this theory never is a human being considered god because a human being does not exist at all and the only real existent is God. Human being is just an epiphany of God.
Detailed Answer
The discussion concerning unity of being is one of the complex topics in mysticism and Islamic philosophy. It requires profound knowledge of metaphysics and Islamic sciences. We are therefore giving an allegorical and simple example of unity. If need be for further study, we will refer you to certain references in this regard.[1]
Mystics maintain that the only real existent in the world of existence is God; others have no real existence but they are the expressions and shadows of God. Therefore, the term "existence" is applied figuratively to them. This statement does not mean denial of God nor does it mean denial of the existence of the universe. Mystics say that they accept multiplicity but multiplicity applies to the expression of existence not to the existence itself.
To explain their views, mystics have referred to Quranic verses as well as traditions which have been discussed in mystical books. Here we shall make mention of one Quranic arguments: The Quran says about God's existence: "He is the First and the Last, and the Outward and the Inward; and He is Knower of all things."[2] This verse indicates that He alone is the Outward and He alone is the Inward. That is when a mystic looks at an ocean, he sees God in it and when he looks at a desert, he sees God in it. A mystic also views the whole universe as the manifestation of the Truth. That is why if a mystic says that when everyone else is in search of God, I am in search of something other than God, it is true because the only one who is manifest is God. Where is that place where He could not be seen?[3]
In order to simplify this point, the mystics have turned to certain examples. Although an example does not and cannot contain all meanings because one has to understand and comprehend it by heart and mind, it can, however, facilitate understanding to some extent.
One of the examples is the example ‘face in the mirror'. Suppose a person is standing beside ten mirrors which are different in terms of concavity, convexity and size. Naturally, he is one person whereas, in fact, there are different pictures of him in the mirrors. This individual has shown ten expressions of himself but these different images or expressions do not cause his essence to be different; rather he is not but one reality with ten different expressions. The conclusion is that multiplicity exists but it is in the expressions not in essence.[4] In other words, those who believe in unity of being never seek to deny the existence of multiplicity. In fact, they are of the view that real existence is that of God. He is the one whose existence is real whereas other beings are just shadows of the real existence. If we look carefully at the mirror example, we will come to know that all the created images are not identical whereas each is different depending on the size and shape of the mirror. Therefore, we do not mean to say by the expression ‘unity of being' that all the existents are one thing and that they are not different from one another. In fact, the stress is on the fact that real existence is that of God despite that other entities have real existence.[5]
It is therefore clear that according to the theory of unity of being the only real existence is that of God and that others have just expressions and shadows of the Truth. Conclusively, multiplicity does exist in the world but it is in the expressions.
There are other examples in the books regarding mysticism and we are not going to mention them for the sake of brevity.
Another point to mention is that this theory does not imply polytheism. That is to say, according to this theory never is a human being considered god because a human being does not exist at all and the only real existent is God. Human being is just an epiphany of God.

[1] If you need to read more, you can read "Kulliyat-e Irfan Islami" by Hamayoon Himmati, Amir Kabir Publications; The Principles of Mysticism and Sufism, Muhammad Hussein Bayat, Allamah Tabatabai University. Also, vide: Answer 31574 (site: 1090) on our website.
[2] Al-hadid, 3.
[3] Lessons on Theoretical Mysticism, Yazdan Pananh, second volume, p. 84 - 85, Encyclopedia of Islamic Rational Sciences.
[4] Ibid, p. 15; also see: The Principles of Mysticism and Sufism, Muhammad Hussein Bayat, p.69.
[5] Lessons on Theoretical Mysticism, Yazdan Pananh, second volume, p. 34 - 35.