A subwoofer is a speaker by itself that you add to a two speaker stereo system. It’s called a 2.1 setup, the 2.0 being standard stereo. Each speaker requires an amplifier channel by itself, hence you need a third mono amplifier for the subwoofer. This is why majority of subwoofers are active with an inbuilt amplifier.
If you let us know your subwoofer maybe we can guide you more in detail.