The world we perceive consists of colors, shapes, sounds, smells, tastes and all kinds of feeling impressions.
Most people assume that there is an 'external' world and that, for example, the eyes are a kind of windows through which we, as it were, staying behind our eyes, look at that external world. And almost everyone used to learn in school that we can see colors with the cones in our eyes. But that whole story is wrong. We "see" color with our brains. Strictly speaking, color does not exist physically, it is an illusion that our brains create to indicate that there is a difference between objects in the environment. This also applies to all other sensory impressions. It is the brain that gives shape, color, smell, etc. to the world. The information that our brains turn into color is brought in via 'electromagnetic radiation', also called 'photons' or 'light particles'. How do we know there are such things as 'photons' or 'sound waves', or 'scent molecules'? These are all the result of interpretations of measurements. The measurements are again communicated to the brain by senses, which then construct an interpretation, which we then learn and process with our brain.
We do not know exactly how this arises and works. There are all kinds of theories. Development psychologist Jean Piaget, among others, has thought a lot about this. And in his work (together with the transcendental idealism of Immanuel Kant) the theory of constructivism has been developed: the idea that knowledge is not there for the taking in an outside world, but is constructed in the brain (or mind if you prefer).
So, according to constructivism, the perception of the world and everything you experience is constructed by the brain. We just can't know what a world outside of the mind looks like, even not whether it exists or not. Even space, time and causality may be constructions and (may) not exist 'really', whatever that may mean :-)
I can imagine that the world is experienced completely differently by a newborn than by an adult, while the baby and the adult have to process the same sensory information when they both look at, for example, a red cube on a white background ...
I remember some time ago teaching computers to people over 80. The desktop of a PC looked completely different to them than to me. For example, I saw all kinds of 'icons'. Not them. They couldn't see any differences in all that color information on the screen. For them it was one flat, chaotic image, in which nothing could be distinguished. This is also how I imagine what it would be like for a newborn to see the world: without separate shapes, without distinction between foreground and background.
So at a certain moment a distinction is made and there is suddenly a distinction between, for example, foreground and background or between areas with deviating colors, or between an 'inner' and an 'outer world'. I can imagine again that this looks like an object suddenly jumping into view when viewing an 'autostereogram', which at first glance contains only chaos or a random dot pattern ... (in the image here you should be able to see a shark – but I never succeed, because of a 'lazy eye').
The fact that it is the brain that shapes the world does not mean that only my brain/mind constructs the world. This would mean solipsism and that would be absurd, because how do I know what words like "brain" or "mind" or "world" actually mean? These words only have meaning within a common vocabulary. So the brain constructs the world in 'dialogue' with other brains. Of course this is not a conclusive proof, because I could of course just believe that what I write here actually means something :-)