What is reality?
Observers come first, and reality as we normally understand it, space and time with everything in them, is the consensus they produce. Two observers can check only what both of them see, so all they can write down is a relation, such as same or different. Collect every such entry into one list, and spacetime with its contents is the single consistent state that fits that list, up to changes no observer could ever detect. There is no further world hiding behind it. A record that survives every comparison anybody can make is a fact, the smallest piece of reality there is. Reality can also be described as a question that answers itself: what would a consistent, self-creating reality be like? The world we live in is that answer, worked out by the observers inside it.
Also in Chapter 5: Why Do You and I See the Same Table?, Chapter 1: Why Is the Universe So Hard to Reverse Engineer?.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read chapter 11: What Does Everybody Actually End Up Holding?watch a universe make its own particles