This page has two external pieces of code added to it that make your cursor sparkle as you move it around the page. One is called a library which is like a big spellbook that has lots of utility spells in it that make it easier to invent your own spells. That one comes from elsewhere on the internet. It's so useful people publish them for anyone to use. If your spell relies on a library that's called a dependency. If you don't connect to the library, or if it changes, your spell won't work.
The next piece of code is hosted on your own site. Instead of
copy-pasting the whole spell on any page we want to use it on, we can
just include a reference to it in the head of the
document.
In both cases, we use a special HTML element called a
script tag. Look at the code on this page to see how it
works.