Why can you follow one friend's voice in a loud cafe and tune out the rest?
In a roaring cafe you can lock onto one voice and let the rest blur into noise.
Selective attention is the brain's ability to pick one stream of sound out of many and follow it while pushing the others into the background. It sorts voices by cues like where they come from, their pitch, and their rhythm, so you can track one friend across a noisy table. This is the cocktail party effect: you attend to one channel and barely process the words in the others.
At a packed coffee shop you follow your friend's story word for word, and the three conversations at the next table dissolve into background hum.
You don't hear everything around you - your brain spotlights one voice and quiets the rest before you even notice.
Knowing your focus is a single narrow beam explains why a loud open office wrecks deep work and why one notification ping can yank you off task.
Cocktail party: one voice in the spotlight, the crowd dimmed to a hum.
Learn the idea and practice English at the same time.