Essentially. To do a full override of the about.hbs template you would add the following to the </head> section of a theme:
<script type="text/x-handlebars" data-template-name="about">
<p>This is my new about page!</p>
</script>
You can put all of the original about.hbs contents in there and edit it down to your liking, or create your own content. Just know that if there is anything important that ever gets changed in core regarding the about.hbs, you’ll need to make the necessary changes to your override.
The link @Johani posted above describes the same thing I just wrote, though 