Subtractive equalization is a technique used by most professional engineers to create clearer, more defined mixes. In order to have a clear mix where all instruments are heard, space will need to be made. Two sounds cannot occupy the same tone or frequency range and maintain clarity. If two sounds do occupy the same frequency range, the louder sound may mask, or hide, the quieter sound. Ultimately, mixing is about “crowd control.” Space must be created for a sound to be heard.
Many inexperienced engineers tend to add what they want to hear first. For instance, if the goal is a bigger, bassier kick drum, a novice may add more bass to the mix. A better solution is to take away from one of the other frequency areas that are dominating the sound, for example, reducing the amplitude around 600 Hz. The result will be more bass on the kick without adding destructive low-end. When mids or highs in the kick drum are cut, more bass will be present.
Also, the area that has just been cut opens up more space in the mix for other instruments to be heard. This is the subtraction in subtractive equalization. This doesn’t mean that frequencies should never be boosted. Start by subtracting first, and then add frequencies only as needed.