Questions specific to stock ROMs (i.e. the "official" ones from manufacturers/providers of the device).
What is a "stock" ROM?
Literally: one from the stock. The term is used with two meanings. In most cases, it refers to "what ships with the device" (or, what is given to you from the manufacturer/provider). Strictly speaking, this is already a modified version of the AOSP ROM (which, to avoid confusion, is also called "Vanilla" or "plain Vanilla"), and thus would be from the "master stock"...
Where should I use this tag?
Whenever your question/issue is truly specific to stock-ROMs as such. Good examples are where you address the pre-installed ROM as opposed to any custom-rom, as e.g. "trouble flashing back to custom ROM".
What are related tags?
For related tags, please see the tag wiki of the 'rom' tag.