唰 is an onomatopoeia that imitates the sound of something moving quickly through the air, like a swish, swoosh, or rustle. It's often used in descriptive writing or speech to convey a sudden, swift motion sound — for example, a bird taking off, a curtain being pulled, or a car speeding by. You'll see it written with the mouth radical (口) because it's a sound word, and it can be repeated for emphasis (唰唰).