If your question is “what is X”, you should look on Wikipedia first. Then, it's fine to ask something like “what is X? Wikipedia says it's a green wobbly widget, but I don't understand what it means for a widget to be wobbly or why it would matter that it's green”. Or “How does X apply to <specific context>”. Or “what is X” when there's a stub-quality Wikipedia article or none at all. Basically anything where the answer would add value to the Wikipedia article, by providing more explanations, or clarifying some points, or applying it to a particular situation.
Another situation where a simple “look at this Wikipedia article” could be a fine answer is when the asker didn't know the name of some concept.
Note that “wikipede it first” is not the same thing as “google it first”. Google answers are fluctuating and unreviewed. Wikipedia articles can change profoundly but rarely do, and are (if of sufficient quality, but this is usually easy to determine even if you don't know much about the subject) cited and somewhat reviewed.