Nonconformity in quality means that something doesn't match the rules for how it is supposed to be. Imagine if you were coloring a picture and you were told to color inside the lines. If you colored outside of the lines, that would be a nonconformity because it didn't match the rules for how the picture was supposed to look.
In real life, nonconformity happens when something doesn't meet the standards that were set for it. For example, if a company makes a product that is supposed to be a certain size but they accidentally make it too big or too small, that product would have a nonconformity.
Nonconformity is not always bad because sometimes it can be fixed easily. For instance, if you colored outside of the lines on your picture, you can simply erase the part that is outside of the lines and try again to make it better. Similarly, if a company realizes there is a problem with a product, they can fix it before it gets to the hands of the customer.
However, if nonconformity is not fixed, it can lead to bigger problems. For instance, if a company doesn't fix a problem with their product, it could potentially harm the person who uses it. Companies want to avoid nonconformity as much as possible to ensure that their products are safe and meet the standards they set for them.