A one-shot is a story that begins and ends in a single part. No chapters, no continuation, no promise that anything follows. The term came out of fan fiction archives, where tagging a work as a one-shot is a courtesy: it tells a reader this will not leave them waiting.
Why erotica is full of them
Because the form fits the reading. A scene with a setup, a turn and an ending works at two thousand words, and a reader looking for exactly that does not want a two-hundred-chapter commitment. Free archives are built around this length, and so is most audio erotica, which is a one-shot you listen to.
One-shot against serial
The business models pull in opposite directions. A serial earns by making you come back, so it ends every chapter on a question. A one-shot earns nothing by that, so it can resolve. Where a platform charges per chapter, the one-shot is the cheapest thing on it, and where a platform is free, one-shots are usually the bulk of what people actually read.
Related words you will see
A drabble is very short, classically a hundred words. A PWP is a scene with the plot deliberately left out, and the acronym is explained kindly or rudely depending on who you ask. A series of one-shots is a collection sharing characters, where each part still stands alone, which is the compromise most writers settle on.
Finding them
On a tagged archive, ask for the tag directly and add a word count filter; that single step removes most of the frustration people blame on the archive itself. In the paid apps the idea barely exists as a label, since nothing in a coin economy wants to advertise the shortest thing in the catalogue, so the nearest equivalent is a collection of standalone episodes.
See also serial fiction and the glossary.