A standard reference is a source (book, website, what have you) that is specifically designed to provide a certain type of information. So, if you're looking for a definition, a dictionary is a standard reference source; if you're looking for a synonym, a thesaurus should be your first destination; etc. So yes, Urban Dictionary is a reference source for current slang terms; TV Tropes is a
black hole ahem, sorry source for looking up reoccurring metaphors in popular media; and Wikipedia is a reference source for things like birthdates of famous people, the list of rulers of a country, or a basic overview of how radios work.
Whether these sources are accurate is a different question entirely. The point is, you shouldn't ask a question on EL&U if there is a type of reference source designed to answer that kind of question, where if you had bothered to look in said reference source, you would have found your answer quickly and easily.
Using the tl;dr example, if this question occurred to you and the first thing you did was ask on EL&U, your question would be quite rightly closed. If instead you looked first on Acronym Finder and didn't find an answer, and you mentioned this in your question on EL&U, then closure wouldn't be quite so automatic. And if you looked also in Urban Dictionary and found a definition that doesn't seem to fit the context where you encountered the term (say some joker defined it as "the law;delicious reading" or something), and again, you mentioned this fact in your question on EL&U, then your question would no longer be general reference.
Note that search engines are not standard reference sources, at least not for the type of information that people can ask about on EL&U. Search engines are a way to look up web pages, and web pages are off topic here.