If your curtains allow light to pass through, they're sheer, or see-through. Sheer can also describe something steep, like a cliff, or anything extreme, like sheer nerve.
This word has several meanings, but they're all extreme. You might want to layer a sheer shirt over a tank top. Climbing a sheer cliff is almost impossible. If your car sheers, it swerves sharply, and you might crash. Maybe you've met all these meanings in a single nightmare: When the sheer fabric blew over your face as you drove, you lost control and the car began to sheer, filling you with sheer terror as you went over the sheer cliff.