The main difference between fixed layout and liquid layout (reflowable) ebooks is that reflowable ebooks always adapt to the screen size. Eg. it doesn't matter if somebody is reading your book from a tablet or from a phone, it will always display properly. The reader can change the font size, and the book will just reorganize itself. Most normal ebooks are reflowable.
In contrary, fixed layout ebooks are a lot like pdf-s: they don't adapt as well to the screen size and look bad on many devices. Fixed layout books are necessary for picture-heavy books (eg. textbooks or picture books) but are a no-go for most fiction or literary works.
Unfortunately only a few stores accept fixed layout publications. You can see a comprehensive list of stores that accept fixed layout books at the stores page. Please switch off the rest of the channels at your 'Stores and imprints' section before sending in a fixed layout book, and only send it to the few stores that accept fixed layout.