Swim caps are typically made of silicone, latex, or lycra and are designed to keep hair out of the face while swimming.
They also help to reduce drag in the water, which can improve a swimmer's speed.
The brand name "PEISO" appears to be a variation of "PARISO," which is a brand name associated with swimming gear.
Swim caps are available in a variety of colors, including the pale pink shown in the image.