In the spiritual sense, attaining nothingness means being open and receptive to the inspiration of the Universe. You are creating a vacuum, which the Spirit can fill. You are not filling this space with your own ideas, preconceptions, cultural conditioning, mental noise, and desperate desires. You are allowing no-thing-ness.
In the quantum sense, nothingness is also the space in which all physical and non-physical interactions occur. As such, it is not really nothing. It is emptiness but it still is full of forces and fields that exert their influence on atomic particles/waves.