Computer aided instruction is when a computer helps you learn something new. The computer can show you pictures, videos, and words to help you understand. It can also ask you questions to see if you understand what you're learning.
Imagine going to school and your teacher has a computer program that shows you pictures of animals, and then asks you what noise each animal makes. The computer can give you instant feedback on whether you were correct or not. This helps you learn more easily because you get to practice and get instant feedback.
Computer aided instruction is like having a helper who can show you things and give you questions to help you learn. It's like having a very smart friend who helps you understand things better.