I’d like to make my and other questions findable using tags (keywords), but I can’t find any guidance on what a tag means. No matter in what context I click a tag name, I only see questions tagged using it - never an editable description of the tag meaning. Is it just not supported?
An individual tag has little meaning, but rather indicates one (of many) areas the question relates to. A group of tags are more likely to provide a sense of meaning. Even so, each question requires at least one of these tags:
- 
meta, for questions about this site (or related to the process of asking a question). - 
common, for questions that relate to all / multiple components in the LO suite. - 
writer, for questions about the word processor component. - 
calc, for questions about the spreadsheet component. - 
draw, for questions about the drawing component. - 
impress, for questions about the presentation component. - 
base, for questions about the database component. - 
math, for questions about the formula component. 
Every now and then I try and clean up some of the more wayward tags, in order to promote a sense of consistency, however this is something that develops over time.
Although the tags do not form a controlled vocabulary (save your and others’ cleaning work), could it be possible to add a bit of explanation, like you did here, except in a more central place?