While frontend is the presentation layer of a software, backend would be the "behind the scenes" of a software. When it comes to Umbraco CMS, we call the backend part of the software “backoffice”.
So you could say that Umbraco frontend is the “face” of an Umbraco-based site. And the way the site is going to look like largely depends on how the Umbraco backoffice is set up.
Umbraco frontend then is what your site visitors are seeing and interacting with. This could be text, photos, videos, other illustrations.
As headless as you need it to be
That’s because Umbraco doesn’t interfere with whatever frontend code you choose to work with and doesn’t mess up the end result.
You can create templates in Umbraco starting with your own markup, that means that the CMS doesn't come in your way and you have a high level of flexibility and creativity. This is exactly why Umbraco is loved by frontend developers far and wide.