The reticular activating system (RAS) is a network of neurons in the brain stem that filters out noise from information. It’s the reason you can hear your name clearly when it’s spoken softly in a noisy room. Your name is important information, so your brain is attuned to it.
Have you ever learned a new word, and then suddenly, it seems like that word pops up everywhere for the next week? That’s the RAS! The word was previously noise, totally filtered out. Now it’s signal, because you just learned it. It has meaning now.
Have you ever gotten a new car, let’s say it’s a Jeep, and then noticed that somehow every car in the world is a Jeep? Of course, the number of Jeeps didn’t change. You just didn’t care. Your brain is being pummeled with makes and models of vehicles all day, every day, so it just filters them out. But when you get a Jeep, it becomes important signal for a little while.
Came to know about from here