One Shot manga is a genre of manga that is published as a single chapter. It's usually one-shot series that were not part of a serialized manga or anime.
One Shot genre manga are simply stories that do not continue on to a sequel. They are the place for the author to try out ideas and tell stories in shorter episodes which can be read in an hour or less.
In the future, these types of short stories will likely be developed into short films or web series with enough interest from readers or investors.