You click the "browser action" icon/button of the extension and it saves a single HTML file that looks exactly like the webpage you have open.
From its FAQ[1] on GitHub:
# What does SingleFile do?
SingleFile is a browser extension designed to help users save web pages as complete, self-contained files. The extension's primary function is to capture an entire web page, including its HTML, CSS, JavaScript, images, and other resources, and package them into a single HTML file.
# I am a web archivist, is it ok to use SingleFile to archive content?
No, SingleFile is not a tool used by professionals to archive content on the Web, especially in the academic field. Professionals prefer to rely on tools based on the WARC specification instead.
[1] https://github.com/gildas-lormeau/SingleFile/blob/master/faq...