Okay kiddo, so you know how when you read a book, there might be some words you don't understand? Like if you're reading a book about science and it talks about molecules, you might not know what that means. Well, the same thing can happen when people read the Bible. Sometimes there are words or phrases that are hard to understand.
That's where something called "biblical gloss" comes in. A gloss is kind of like a little helper that explains what a word or phrase means. So if you were reading the Bible and you saw the word "atonement," which means making up for something you did wrong, but you didn't know that, there might be a gloss that would explain it to you.
Now, sometimes glosses are already written into the Bible. This means that when the Bible was translated into a different language, the people doing the translating would add little explanations right in the text to help people understand it better. But other times, people might write their own glosses to help them (or someone else) understand the Bible better.
And that's what biblical gloss is - it's just a fancy name for those little explanations that help people understand the Bible better!