This means it is a characteristic, or property, of knowledge rather than of reality. What is meant by something being a property of the observer rather than of the observed. In our modern society, the use of probability to describe events is pervasive. What's the chance of rain today. We use probability so much that we forget that probability does not describe the event being observed.
Probability describes our knowledge of the event.
For further discussion of the relationship between knowledge and reality see The Nature of Knowledge. Not at all. To say spatial dimensionality is a very powerful tool may be one of the all-time greatest understatements. However, if spatial dimensionality is a property of our knowledge, then it is not a fixed universal truth. I believe that it has.