EPUB is an "industry-standard" ebook format. I don't use it much. You don't need to know about it to create a Kindle book.
Wikihow on converting a Word doc to EPUB
Here's some info on EPUB from Wikipedia
"The EPUB format is the most widely supported vendor-independent XML-based (as opposed to PDF) e-book format; that is, it is supported by the largest number of e-Readers, including Amazon Kindle Fire (but not standard Kindle).
"EPUB is an e-book file format with the extension .epub that can be downloaded and read on devices like smartphones, tablets, computers, or e-readers. It is a technical standard published by the International Digital Publishing Forum (IDPF). The term is short for electronic publication and is sometimes styled ePub. EPUB became an official standard of the IDPF in September 2007, superseding the older Open eBook standard. The Book Industry Study Group endorses EPUB 3 as the format of choice for packaging content and has stated that the global book publishing industry should rally around a single standard. EPUB is the most widely supported vendor-independent XML-based (as opposed to PDF) e-book format; that is, it is supported by the largest number of hardware readers.
"It is also widely used on many software readers such as iBooks on iOS and Google Books on Android, but notably not by the Amazon Kindle e-readers. iBooks also supports the proprietary iBook format, which is based on the EPUB format but depends upon code from the iBooks app to function.
"EPUB 2.0 was approved in October 2007, with a maintenance update (2.0.1) approved in September 2010. The EPUB 3.0 specification was published in November 2014. New major features include support for precise layout or specialized formatting, such as for comic books, and MathML support."