Kindle Unlimited is a subscription service offered by Amazon, which allows you to access as many stories as you like for a monthly membership fee.
Publishing stories on Kindle Unlimited means authors are forbidden from making titles available in stores other than Amazon. For this reason, I publish only a limited range of short stories to Kindle Unlimited, so that I can offer most of my stories to readers using Apple Books, Kobo, Nook and other e-reader devices.