What are 2 white disc shaped things at bottom of pool?

At the very bottom of the pool are the bottom drains . By regulation , they must be 36" apart . Either of these drains must be able to support the total suction of the filtration system . Theis is the Virginia Grahme Baker regulation . At other places on the pool bottom you may see smaller discs . These are the returns , the outflow of the filtration system .