A string trimmer is a tool that helps cut down weeds and tall grass. It has a long stick that you hold with your hands and on the bottom of the stick, there is a thing that spins around really fast. That spinning thing is a string, and when it touches the weeds, it cuts them down.
Sometimes, when you use the string trimmer, the string can get too long and get stuck on things like rocks or trees. To fix this, you can "trim" the string by pressing a button or pulling a lever to make the string shorter. This way the string will be less likely to get stuck, and you can keep working on cutting down the weeds. Some string trimmers also have different types of strings to help cut different types of weeds.
Remember, whenever you are using a string trimmer, make sure to wear safety gear like goggles and gloves to protect your eyes and hands. And always ask an adult to help you use the tool!