K-12 means kindergarten to twelfth grade, and it's the system that the school uses to teach kids like you. So when you're in kindergarten, it's your first year of school, and you're there to learn all the basic things like the alphabet, numbers, colors, and shapes.
When you move up to first grade, the teacher will keep teaching you new things, like reading short books and adding numbers. As you keep moving up the grades like second, third, and fourth, you'll keep learning new information, and the subjects will get more challenging.
Once you reach middle school, which is usually sixth to eighth grade, you get to experience different classes like science, math, history, and art. And as you get closer to high school, which is ninth to twelfth grade, you'll get to choose more of the classes you want to take, and even have the chance to attend a vocational school to focus on specific skills like construction or nursing.
Overall, K-12 is just a way of saying that school is a really big part of your life, and you'll keep learning new things every year until you graduate from high school.