Why can you keep talking to a stranger who was secretly swapped for someone else?
A large change can happen right in front of your eyes, and if it hides behind a half-second interruption, you'll miss it.
Change blindness is our failure to notice a big change to a scene when it happens across a brief interruption - a blink, a camera cut, a moment when something blocks the view. A visible change normally grabs us because it creates motion. Take away that motion signal and you're left comparing the new scene to a memory of the old one, and that memory holds far less detail than it feels like. So the change slips by unnoticed.
You glance up from your phone, a coworker has swapped seats with someone else while you looked down, and you carry on the chat without registering that the face changed.
You don't store a full picture of a scene - so a big change hidden behind an interruption can pass you by completely.
It is why continuity errors slip past film editors, why "spot the difference" is hard, and why eyewitnesses can miss a swap they would swear they'd have caught.
No motion to catch your eye plus a thin memory equals the change you never saw.
Learn the idea and practice English at the same time.