To skip over quite a bit of philosophy, I accept that there is a real, physical reality out there. It is composed of an expansive universe filled with physical matter, following laws discernible by science. Even though I accept that this is the case, when humans perceive the world, we are not truly seeing this objective reality.
Our brains are vastly complex information processors that seek to make sense out of the world for our survival. Doing this includes making assumptions, constructing models, and making subconscious jumps in reasoning that may or may not be an accurate reflection of reality.
Take the picture below as an example. I will let your mind be blown before we move on:
The squares are, objectively, the same color, so why do we perceive the top one as darker and the bottom one as lighter? To put it very simply, the intersection of the two squares triggers our visual system, which in turn makes assumptions about what we are seeing. The apparent 3D perspective (enhanced by the separated background giving the impression of distance) and boundary between the squares both tell our brains to adjust what we are seeing, as the brain is constantly adjusting what we see according to lighting and depth. As a result, the squares appear as different shades. We know that the world is not homogeneously well-lit, yet it is an assumption our brain makes, and this allows us to distinguish colors and shades in different lighting schemes. The picture above is taking advantage of these assumptions.
These assumptions are also the reason why putting your finger over the boundary between the two squares eliminates the confusion. Without any triggering of our assumptions about lighting and depth, our brain then sees the squares as they truly are (as having the same color).
Illusions like these highlight just how much our brains contort reality to fit our evolutionary needs. And if anything, they are always great examples of why you can’t trust everything you see.