And discrimination tells you whether doing well on the question correlates with doing well on the test. A question with low discrimination is probably not performing a useful purpose. Especially if you are doing large scale testing, these statistics give you really valuable information. If you're making multiple choice questions, word the distractors carefully. Don't make it possible to guess what is right just by common sense examination of the answers.
Common errors are to make the right answer the longest answer or the most carefully worded one.
Or to include some distractors that are obviously wrong. The best question has four or more answers, each of which could be right. If an alternative way of phrasing the answer is correct, then either make sure it is treated as correct, or change the question to make it clear what answer you expect. Unless you know what you are doing, keep screen layout simple. Use a small number of fonts and colours - Arial works well on Windows.