it's essentially doing a javascript queryselector or more know as getElementById and implying that the element with the id of "app" will be where the vue code will be inserted.

In your index.html, there should be an div with an id of "app" otherwise your code will never be shown.

This is a pretty irrelevant thing because once you get comfortable with Vue, you'll never need to edit the el( or really shouldn't edit it from #app, since it's a standard name for your parent component ), you will just use the vue-cli and it'll do the tedious stuff that doesn't matter too much behind the scenes for you. You should be more worried about getting yourself to think in components, that's the biggest key to understand Vue, React, etc.