Chess Stack Exchange is for serious players and enthusiasts of chess.
Questions that have to do primarily with chess in its standard form are on-topic here. This includes questions about theory, rules, specific positions/games (including puzzles), chess-specific events, chess players, hard facts such as statistics and other historical data, and physical items such as chessboards, pieces, and clocks.
Other, less specifically chess-related subjects may also be on-topic, depending on how chess-related the specific Q&A is. These gray-area questions include topics of etiquette, psychology, software, books/articles/media, as well as other subjects. Chess-specific questions about programming a chess engine or other chess software are welcome.
In general, questions which can be answered without significantly referencing any chess-specific data (other than whatever data was presented by the question) are off-topic. Chess-specific data means data that would be directly invalidated without reference to chess.
Questions about variants may or may not be accepted, depending how far removed from the standard game it is. In general, a variant which uses the same board and pieces as standard chess is an on-topic subject.
If your question is not specifically on-topic for Chess Stack Exchange, it may be on topic for another Stack Exchange site. If no site currently exists that will accept your question, you may commit to or propose a new site at Area 51, the place where new Stack Exchange communities are democratically created.
For more help, see "What types of questions should I avoid asking?"