<script src="Tone.js"></script>
<script>> is the element that is used to load JavaScript files
src attribute tells us that we are loading JavaScript from a separate file...
... and we give the filename as the attribute to it. So, in this case, we expect a Tone.js file to exist in the same directory.
When loading external files with JavaScript it's a good practise to also close the element, even though there's no actual content inside it.