Okay kiddo, so you know how your body has a system called the "sympathetic nervous system" that helps you deal with stress or danger? It makes your heart beat faster, your muscles tense up, and your breathing gets quicker so you can fight or run away.
Well, there are some medicines called "sympathomimetic drugs" that can help your body do those things even when you're not in danger. They "mimic" or copy what the sympathetic nervous system does.
For example, some people use a drug called "albuterol" to treat asthma. It works by making the muscles around your airways tighten up less so you can breathe easier. This is the same thing that would happen if you were running away from a bear!
But, these drugs can also have side effects because they affect your whole body. They might make your heart beat too fast, or make you feel shaky or nervous. That's why doctors only use them when they are really needed and monitor your body carefully.