What the manual is about. What your manual covers, and more importantly, what it doesn't cover. If appropriate, tell them where to go for additional help. This could mean on-line help, other manuals, or other experts. No introduction includes all of these items.
Some introductions are quite short, others are very detailed.
Put your topics in order of importance for your manual, not in the order that I've presented them here. This function can be very important in a long, complex manual. Readers often get lost at the transitions between major sections. You need to hold their hand through the process. In a complex manual, you often have to include orientation information in a summary at the end of one chapter and again at the beginning of the next.