Right now you are using your muscles, in fact there has never been a time in your life when your muscles have been idle. In your body there is over 600 muscles all working together to let you blink, breath, and live. Your muscles need oxygen, nutrients, and protein to even function. Thats why body builders eat healthy so their muscles can grow faster and stronger.
When you are just a toddler your muscles are not strong enough to lift a 100 lbs weight. Even If you were 30 and you did’t work out your muscles you probaly would’t be able to do that either, but if you trained your muscles(some kind of pyshical exercise often) it would be extremely easier and most likely you would be able to lift more then 100 lbs.
The most important muscle is the most vital one and the key that keeps you alive. It’s you heart. Without you heart you would not be able to live, and therefore without your muscles you would die. There are two main kinds of muscles. Voluntary and Involuntary. Your Voluntary muscles are the ones that you have to think about before you use them. For instance when you jump you are telling your body to jump. Involuntary muscles however are more important like you heart, which you don’t have to think about breathing.
This is just a cheap essay that explains things you take for granted.