A stub is an incomplete article that the original author thinks does not meet a minimum of useful information.
You can find the stubs in the stub category.
You can mark a page as a stub with the generic template {{stub}}, which produced the following text:
This alternate history related article is a stub. Please add suggestions on the [[Talk:Alternative History:Stub|talk page]].
Or you can create a template of your own, using a pre-existing stub template such as Template:stub, Template:BLstub or Template:Ddstub as a model if desired.