Open an .epub ebook
An .epub is the ebook format, but double-clicking one usually does nothing: the computer does not know what to open it with, and the apps want to be installed and signed into. Here you open it and read it straight away, with the chapter list and text you can enlarge. The book stays on your device.
Why an .epub does not open on a double click
Because it is not a document, it is an archive: inside are the pages in HTML, the images, the fonts and an index saying in which order they are read. Browsers have no built-in reader, and the programs that do open it often want an account. Here the archive is opened in the browser and the pages are put back together one after another, with their titles. It works with modern epubs (EPUB 3) and with old ones (EPUB 2), whose index is written a different way.
DRM, and how it differs from protected fonts
If you bought the ebook from a shop with protection, the text inside the file is encrypted: without the key no program can open it, and this page tells you so instead of showing you a blank page. That is not a limit of this tool, it is how DRM works. Do not confuse it with obfuscated fonts: plenty of perfectly healthy books protect only the fonts, and those read just fine, so here they are recognised and the book opens anyway.
Images and links stay inside the book
Figures and the cover are pulled out of the archive, so they show without a single request to the internet: nobody can learn that you are reading that book. Links between chapters work and jump to the right place; links that lead out of the book open in a separate tab, and only if you click them.
The text is text, and you can take it with you
It is not a photograph of a page: you can select and copy, enlarge the font if your eyes need it, and with one tap save the whole book as a text file, chapters one after another. That comes in handy: to search it properly, to paste a quotation, to read it on an old phone or to have the computer read it aloud.
What it does not do
It does not remove DRM and does not get around any protection. It does not convert to PDF or mobi. It does not keep your place: close the page and it starts over (the book is not stored anywhere, and that is exactly the point). If the file you have is a .docx, a .zip or an .eml, there are pages made for those.