Nirguna brahman is subtler than the subtle state. By the work of MAyA the concrete creation takes place. This is the concrete state.
In between the two states is the state of Hiranyagarbha. This is the state where creation has not yet taken place, but the saguna-brahman with its MAyA has kept the whole creation within itself as if in the embryo stage.
Hiranya means gold. AvidyA (Ignorance), otherwise MAyA, by itself is like darkness, but by the presence of Brahman-consciousness it works out this wonderful task of creation, the consciousness which thus shines and reflects is said to be golden.
The gate that allows things to go out is also the gate through which things enter. So the creation which came out from Hiranyagarbha goes back inside through the same Hiranyagarbha.
When does it go back? – when Hiranyhagarbha is of age one hundred and thus his lifetime is over, he merges into nirguNa brahman.