Subtractive synthesis and additive synthesis are two different approaches to sound synthesis. If you look at the technical specifications of a hardware synthesizer, it will usually be distinctly labelled as one of these two (granular and wavetable synthesis come later as features of primarily software synthesizers).
Subtractive synthesis involves starting with a harmonically rich waveform, such as a square or sawtooth wave, and then using filters and other processing tools to remove frequencies from the sound. This process is called “subtractive” because it involves removing parts of the original sound to create a new sound. Subtractive synthesis is often used to create classic synth sounds and is commonly found in analog synthesizers.
Additive synthesis, on the other hand, involves starting with simple waveforms, such as sine waves, and then combining them to create more complex sounds. This process is called “additive” because it involves adding together multiple waveforms to create a new sound. Additive synthesis is often used to create sounds that are difficult or impossible to create with subtractive synthesis, such as bell-like tones and other complex sounds.
Both subtractive and additive synthesis have their own unique benefits and drawbacks, and many modern synthesizers incorporate elements of both techniques.