When your vehicle is running, you have a number of working parts that contribute to how efficiently and comfortably everything is working. When your engine is running, this produces a lot of noise. Rather than having all of this noise taking over the cabin of your vehicle, the muffler will muffle this noise and expel it out the back of your vehicle. Some vehicles with large engines come with two of these mufflers to get the job done right.
The exhaust portion of your vehicle is used to decrease the amount of harmful gasses that your vehicle is putting out into the environment. The muffler is actually the very last part of this exhaust system.
You'll want to make sure your muffler and exhaust are working properly. If something is wrong, watch out for a strong odor or loud noises while you're engine is running.