Bleed and safe area: where your artwork goes, and where your title must stay.
For a KDP paperback cover:
-Bleed: 0.125 in added to the top, bottom and outside edges. Backgrounds and images must run all the way to the bleed edge, because that strip is trimmed off.
For a KDP paperback, spine width depends on your final page count and paper type. For example, a 246-page book on white paper with a black and white interior comes to about 0.554 in (246 x 0.002252 in).
A sample project: matching covers for a cozy mystery series.
Two Kindle ebook covers, one consistent look across Book 1 and Book 2, with titles that stay readable at thumbnail size. A self-initiated sample project, not client work.
If you are planning a series, I can design all the covers to match.