Okay kiddo, have you ever looked at a picture that was a little blurry and hard to see? That's because the resolution wasn't very good. When we talk about resolution in science, we're talking about how clear we can see something at a very small level.
Electron density is all about how tightly packed electrons are in a material or substance. Those little electrons can be hard to see because they're so tiny, but we can use special machines to measure how many electrons are in a certain area.
When we combine these two things - resolution and electron density - we can get a really good look at what tiny things look like. Think of it like taking a really good magnifying glass and looking at a tiny bug up close. With strong electron density, we can see everything much more clearly and understand it better too.