Okay kiddo, you know how when you go to the store to buy a new toy, you look at it to make sure it's not broken or doesn't have any scratches or missing parts? That's kind of like what non-functional testing is for computer programs and websites.
Non-functional testing is when people test software or websites to make sure they work properly in ways that aren't related to what they actually do. So, it's not about making sure the toy does all the cool things it's supposed to do, but about making sure that it works well in other ways.
This means that for non-functional testing, the testers are looking at things like how fast the website loads, how easy it is to use, how many people can use it at the same time without it crashing, and how secure it is from hackers or other bad people.
So, just like how you want to make sure your toy is in good shape before you buy it, non-functional testing helps make sure that computer programs and websites are in good shape before people start using them.