In general, it's a small website built to complement an existing website/set of websites. Traditionally, these have been used for Instagram/Twitter profiles to fit more than the one allotted link in a bio, but our users use them for a so much more! (ex: internal team bookmarks archive, simple product page/checkouts, adding a paywall to a small page/item, or simply using them as their website for ease of use)

Hope I explained myself well! :)