Your brain receives constant messages about your body and the external environment from the sensory organs. That’s your eyes, ears, nose, tongue, skin, and so on. With this information, your brain maintains a 3D map of your body and keeps a tab on what’s happening outside your body.
How your brain sees a situation may not be entirely accurate.
Perception of Reality
Sometimes your brain even ‘fills in the blanks on your behalf, and your experience isn’t 100% based on reality but is instead a perception of reality. An interesting way to demonstrate how the brain’s inner reality is a perception is using the classic checker shadow illusion, created by Edward H Adelsen from the Massachusetts Institute of Technology.