This is for computer database searches to pick up on, along with words in the title and abstract. So think about what kind of search items you would want leading to your paper. This goes along with finding a Math Reviews code, which can be included in a foot note if you know it. So it should be beautifully written with much work. Up to a quarter of the manuscript might reasonably be taken up by the introduction and the preliminaries.
Most people find it easier once they get onto the mathematics itself.
You should try in the introduction to cover the following checklist. This should recall to the reader why the kind of result mentioned already in the abstract would be interesting and important. It also tells the reader what you think is the motivation, so that if he or she agrees with the way you are looking at the field, there's some probability that the paper will be useful for them. Keep it as down to earth as possible. Don't just repeat the abstract.