A reflowable eBook format means that the text of the eBook will "reflow" to fit the screen size of the device that it is being read on. This means that the text will be re-wrapped to fit the width of the screen, and font sizes can be increased or decreased to make the text more comfortable to read.
There are a few different eBook formats that support reflow, but the most common is EPUB. If you have an eBook in the EPUB format, you can open it on most e-readers and the text will reflow to fit the screen.
Kindle devices do not natively support the EPUB format, but there are a few workarounds that will allow you to open EPUB files on Kindle. One popular method is to convert the EPUB file to the MOBI format, which can be done using a free online converter like Zamzar.