What is an imprint?
An imprint is the publishing name your book is released under — the "publisher" brand on the copyright page and spine. Big houses have many imprints; as a self-publisher, you can simply create your own (e.g. "Riverbend Press") to give your books a consistent, professional publisher identity.
It's entirely optional — plenty of authors publish under just their own name. If you do use one, keep it consistent across your books, and add it in the generator's imprint field. Note that using a free KDP ISBN lists Amazon as the publisher of record regardless — to show your own imprint as publisher, you generally need your own ISBN.
Two optional extras sit with the imprint name. A website, which prints on the copyright page and makes your mark tappable in the ebook — see Links in your ebook. And a publisher's mark, the small device on the title page. Neither is expected of a small press, and an imprint name on its own is entirely conventional.