at $T approx 0 , text{K}$, will all energy levels within the electronic band structure be occupied up to a certain level?

I saw this from the script of my teacher that I don't understand what does it mean

If we cool down a crystal to an absolute temperature of T ≈ 0K, all atoms of the crystal will exist at their ground states and all energy levels within the electronic band structure will be occupied up to a certain level

At the absolute zero, all electron is at its lowest energy level so the higher level is empty so why all energy levels within the electronic band structure will be occupied up to a certain level. I don't understand what it means. The higher level is empty so why all energy level (including higher level) is occupied at a certain level?