An ePUB reader lets you read a book that was published as an ePUB on electronic devices, such as smart-phones, tablets, computers, and so on.
See https://docs.bloomlibrary.org/ePUB-notes to learn about the performance of various ePUB readers.
Both fixed and flowable ePUB modes are considered there.
You may learn from the community which ePUB readers work best in various situations and the kinds of books you publish.