A reflowable layout is exactly what it sounds like, a format where the contents of the page can “flow” in order to automatically adjust to the device's screen size. The lines of text and pagination will change in order to match the device and its orientation.

What is the difference between fixed and reflowable format?

The main differentiating factor is that reflowable content changes its placement according to different screen sizes; while the fixed layout content does not. So, if the author has a PDF or a manuscript which mainly consists of text, it's better to go for a reflowable format.

How do I make my eBook reflowable?