Bit depth refers to the size of the binary digit used to store a color value. The higher the number, the more granular the value stored.
Bit depth is not dynamic range. In other words, your blackest black and whitest white do not change with bit depth. The “range” of colors doesn’t even necessarily change. The effect of bit depth is in the amount of subtle differences between the darkest and brightest point of any given color.
Imagine it like a staircase. Increasing bit depth doesn’t move the first floor any lower or the second floor any higher, it simply adds more steps between the levels.
As always, Frame.io has a great explanation.