In a pool what is the white circle thing in deep end?

It is a safety feature that prevents swimmers from getting sucked into the pool's drain. The drain cover is usually made of plastic and is about 12 inches in diameter. It has a hole in the center that allows water to flow into the drain, but it is too small for a human body to fit through.

Drain covers are required by law in most countries, and they are an important safety feature for swimming pools. They help to prevent drownings and other serious injuries.