"What's the context?" is one of the most common types of comment you may get when you post a question about some expression on this site. The Japanese language depends heavily on the context. Many words and phrases have more than one meaning or translation; even a simple phrase such as 彼が好き can mean "He likes it", "I like him", and many others. Many words, even including subjects, can be safely dropped if they can be inferred from previous sentences. Creative writers may use a word in some special way. Japanese relative clauses are inherently ambiguous (see: ambiguous-relative-clauses), and their meanings often purely depend on the context.
Usually, it's best to copy and paste three or four original Japanese sentences around the sentence in question. If it's from a manga, consider copying the original page as an image. You may think that's too bothersome and time-consuming, but that's often the fastest way to get a correct answer.
If you still have doubts, please read on.
Native speakers can make mistakes due to the lack of the context
Even native speakers have made terrible mistakes in their answers because there was not enough context. They usually don't post an answer if there isn't enough context and certitude, but sometimes they make a wrong assumption about the context. Here are examples:
(Of course other native speakers made mistakes, too, but most of the wrong answers by them have been deleted already.)
Some expressions are simply unfamiliar even to native speakers
If you've already looked up the expression you don't understand in the dictionary and you're still unsure, then maybe that expression is difficult even for average native speakers. If it's a well-known net meme or a parody of something, you'll get an answer instantly. But what if it's not? Are you sure it's not a coined word or a word used only in a niche community? Several people have asked about the meaning of a word without realizing it's used in only one work:
Original text is better than your own interpreted context
Some questioners try hard to explain every context in English, leaving only one sentence (or a part of a sentence) in Japanese. However, this can sometimes be counterproductive. Instead, please try and provide as much original Japanese as possible. For one thing, your interpretation of the previous context may not be 100% correct; the actual source of your confusion may be in the previous sentence. And for another, the original Japanese text very likely contains lots of untranslatable information such as role language (役割語{やくわりご}), the overall politeness level, and the tendency of kanji choice and personal pronouns. Experienced readers can often make out the personality of a character accurately just from a single sentence, which may be important even when your question is not directly related to dialects or writing styles.
Some people try to change the context, alter the original sentence, or even make up an entirely new context, believing it would result in a more "focused" and "self-contained" question. Please don't do this—it can instead result in messy comments just to clarify the actual context.
Clarify what each pronoun refers to
When there is a pronoun or a similar phrase (それ, 彼, あいつ, こんなの, そういうわけで, etc.), be sure to clarify what it refers to. Usually you can do this by providing the original text before the sentence in question, but there can be exceptions.
Always provide the title if you can
Please always try to share the title of the material you are reading (preferably with a link, the page/episode number, etc.). Sharing the title is the most cost-effective way to provide the context, because with it people can easily search for the synopsis and the setting if they need to know more about the background. Many works can be read in full online today. This will save time for both the questioner and the answerer.
You don't have to worry about overwhelming people with too much information. Many native speakers can read Japanese text much faster than you might imagine, and to them, asking for the context and waiting for hours may be more frustrating. It is important that questioners avoid playing a game of twenty questions with people trying to help. In edge cases, one may have to read an entire chapter or Wikipedia article to get the meaning of only one expression, but even this may be better than asking for the context and waiting. When your question is about song lyrics, this is even more important; the interpretation of a single word may rely on the full understanding of the entire lyrics.