Reliable sources are exactly what they claim to be: sources of information that can be considered to be reliable.
All articles require reliable sources. Otherwise, there is nowhere to get information from. Please do not edit using purely information out of your own head. That is called creative writing, and there are much better places to do that than here.
That is not to say that you must cite every single source you use, or even that you have to give a citation for most of the material you provide. General game information, such as item statistics or map data, does not require you to cite anything. However, if another editor questions one of your edits, it's best to provide a screenshot or other way of verifying that your information is valid.
Articles should rely on reliable, published sources with a reputation for fact-checking and accuracy. Information from sources such as MAIET and ijji can be considered to be reliable sources, because as they are responsible for the game, whatever they say is true. This means that we only publish the opinions of reliable authors, such as GMs and developers, and not the opinions of editors who have read and interpreted material for themselves. Proper sourcing always depends on context; common sense and editorial judgment are an indispensable part of the process.
The term "published" is most commonly associated with text materials. However, audio, video, and multimedia materials that have been recorded then broadcast, distributed, or archived by a reputable third-party may also meet the necessary criteria to be considered reliable source. Like text sources, media sources must be produced by a reliable third-party and be properly cited.
How Will You Know?Edit
Determining whether something is a reliable source demands logic. Everything is subjective. For example, if a developer for a private server says something about his own server, he can be used as a reliable source. However, if the same individual is making claims concerning an official server, those statements cannot be considered reliable.
When Reliable Sources Are HelpfulEdit
Reliable sources are almost always very helpful. Information provided by a reliable source can be grounds alone for something to achieve notability, although this is by no means automatic. Sources allow editors to make claims that would otherwise be considered to be pure hearsay, rumor, or opinion. When two editors disagree about a topic, whichever one has reliable sources will inevitably always win.