Whether you know it or not, every stereo has an amplifier. Your speakers, tweeters and subwoofers (if you have one) all need power. Amplifiers are what power these components of your stereo. Without them, there would be no sound.
Let's start with the understanding of an amplifier, what it does, and why you need one. First off, in any audio system (car, boat, home, RV, etc) an amplifier is the component that takes a weaker signal and amplifies it. Think of a megaphone. There's the input (your voice), that meets the amplifier (the microphone + power), and the output (the speaker). In this scenario it takes the weaker signal (your voice) and turns it into something that's louder and clearer using a microphone and power. Similarly, an amplifier in a car or boat will take a weaker signal and amplify it into a stronger signal that can be used for things like speakers or subwoofers.
Every audio system has one, whether it's a stand alone component that's under your car's seat or built into your car stereo. When you're looking to buy a car stereo, you'll notice that every unit has an output, typically it's something like 4 x 50W Peak. This means that the head unit's amplifier has an output of 50 watts per channel, times 4 channels.
Do You Need An Amplifier for Your Subwoofer?
In nearly all cases, the answer is yes. Unless you have a system in your car or boat that is already powering a subwoofer, you'll need to find an amplifier that will power your sub. Your head unit will typically provide a low-power signal (RCA output) that can be used for an amplifier to power your subwoofer. But without the amplifier, you basically have a 2 or 4 volt (sometimes higher) system and that's not nearly enough to power a subwoofer. Subwoofers take a lot of power and, depending on their size, may even require a dedicated amplifier of their own.
If you're looking to add a subwoofer to your system, take a look at our article that breaks down what to look for in a subwoofer amplifier. If you already know what you're looking for in terms of an amplifier, you can check out our editor's top picks for subwoofer amplifiers as well!