Okay kiddo, you know how sometimes we have a really long list of things to do, like clean our room, do our homework, and walk the dog? It can be hard to decide which thing to do first, right? That's kind of what listing priority number is all about.
Sometimes grown-ups have a lot of things they need to do for their jobs, and they need a way to figure out which thing is the most important. So they give each thing on their list a number, starting with the most important thing being #1, and the least important thing being #10 (or however many things are on their list).
That way, they know they need to do the thing with #1 first, before they can move on to anything else. It's like a special order that helps them stay organized and make sure they don't forget anything really important.
Does that make sense, kiddo?