ePub is a file format developed for eBook readers. The advantage is that the display of the content adapts to the device used. The ePub format is therefore particularly suitable for small devices such as smartphones or e-readers. The ePub format also offers the possibility (depending on the end device) to adapt the font and font size to individual needs.
Of course, you can also read a book in ePub format on a device with a large screen, such as a computer or laptop. For this you need special free software, such as Adobe Digital Edition or Calibre.