Subtractive colour is based on light being absorbed by a surface, and the light that isn’t absorbed is reflected as a colour.
The Primary colours we use are Cyan, Yellow & Magenta. If you coat a white surface equally with 100% of Cyan, Yellow & Magenta then we see black and where there is no colour we see white.
In printing we also add Black to achieve better grey shading than what can be achieved with CMY alone. Black also allows us to make a darker colour than we can produce with just CMY.