ANSI/ISEA 110-2003 is a set of rules that tell grown-ups how to make sure that children are safe when they wear clothes that are meant to protect them, like hard hats, safety goggles or gloves.
Think of it like this: when you're going to play outside, your mom and dad make sure you wear a helmet when you ride your bike so that you don't hurt your head if you fall off. The grown-ups who make these helmets have to follow certain rules to make sure that the helmet will really protect you from getting hurt.
That's what ANSI/ISEA 110-2003 is for. It tells the grown-ups who make the clothes you wear when you're doing things like building, fixing or gardening what they need to do to make sure you're safe while you're doing those things.
It talks about things like making sure that the clothes are the right size for you so that you don't trip or get tangled up, or that they're bright enough so that people can see you if you're working outside in the dark.
So, basically, ANSI/ISEA 110-2003 is a set of grown-up rules that make sure that the clothes you wear to protect you when you're doing fun things outside, won't actually hurt you or make you less safe!