In order for any idea or material thing to be analyzed, i.e. understood with the mind, there has to be an abstract representation of it. And as soon as an abstraction is created in our minds, there has to be a complement of the abstraction, in order for the abstraction to exist.
For example, let's say we want to analyze a pencil. We will first create an abstract representation of the pencil in our minds, this can vary from person to person, but they have an abstraction for the pencil in their minds, nonetheless. When that abstraction matches with an object, they call it a pencil. Now, the complement of the pencil is the absence of pencil, there has to exist at least one abstract representation of the absence of the pencil. Maybe it is the pencil-sized emptiness in the pencil box, maybe it's the empty space on the desk. But there has to exist a representation of the absence of the pencil in our minds in order for it to detect a pencil. In other words, the origin of a pencil's presence is from its absence, and vice versa.
We can replace 'pencil' in the above example, with any material thing or idea, even the whole reality we perceive with our minds. The whole reality we perceive with our minds needs to have a complement, which makes us perceive the reality as it is. Perhaps the force which doesn't let us understand the complement of reality is what is spoken about in Advaita Vedanta.