Why am I here?
If you were sent here, somebody thought that you posted your question too early, that is you don't know facts and/or techniques one would learn in undergraduate courses. This is not your fault, but for us who write answers it has become a chore to explain the same things over and over again (many such questions arise from homework, and homework problems do not differ a lot).
Therefore, we would like you to go over your course material again (if you are a student) or peruse material readily available in libraries or the web. Chances are you will be able to answer or at least improve your question with your new knowledge!
In order to help you with that, we have compiled a list of questions that are of general nature and have answers that should apply in a variety of situations. Please take the time to browse through those relevant to your question; chances are that we already have you covered. If not, it is likely that one of two things has happend:
You don't understand the reference material.
In this case, the best response is to do further research. That includes picking up textbooks (the reference answers may list some) and asking focused questions on the main site.
You understand the material, but you can not apply it to your situation.
In this case, edit your old question to include your attempts at solving the problem and why they failed. Then flag it for reopening; with this new information, we can help you identify your specific problem and move forward.
Remember: "I don't understand any of this, please explain in plain English!" is a bad enquiry. Nobody can know what your problem really is, which factoid would help you understand, and what the required scope for a good answer is. Try instead to phrase questions like "In above proof, why does B follow from A?" or "I have an algorithm but it seems to be wrong for corner case X, how can I fix it?"
That said, you find links to the reference posts grouped by topic below: