A sheer is a thin, semi-transparent fabric that softens daylight without blocking it. It makes a window feel layered and considered.
A sheer curtain is a thin, semi-transparent fabric, typically a fine polyester voile, linen voile, or lace, that lets daylight pass through while softening it. You can see the texture of the fabric clearly in light. From outside in daylight, the sheer breaks up the line of sight enough to feel private. From outside at night with the lights on inside, you can see in.
Sheers are a styling layer first and a functional layer second. The most common Singapore use is as the front layer of a sheer-and-blackout pair in a bedroom, where you draw the sheer during the day for a soft daylit feel and pull the blackout at night.