Okay kiddo, do you know what computer programs are? They are like instructions that tell your computer what to do. But some programs are very big and can slow down your computer. That's where lightweight software comes in.
Lightweight software is like a mini version of a program, it only has the most important instructions and doesn't have the extra stuff that you don't need. It's like having a backpack with only the things you need for school, instead of carrying a big heavy suitcase with stuff you don't need.
For example, there are lightweight web browsers that let you go on the internet but don't take up a lot of space on your computer or slow it down. They still let you do what you need to do, but they don't have all the extra features that some people might not need or use.
In short, lightweight software is like a lighter, faster and more efficient version of regular software that's easier to use and doesn't take up as much space.