They usually give you a warm welcome to try to alleviate the fears of both parents and the children. Then they move on to what is going to be basically be taught and what to expect and possibly some of the rules of the school. It's kind of a rundown of everything that will transpire and be expected in the school year. Also, they will throw in some goals and probably say they will want you to volunteer for any special projects that the individual teachers may need assistance for during the course of the year. I found if you volunteer for some of the things you really enjoy doing you won't feel obligated to those you really can't. My kids school sent around a paper with all the projects for the class that needed volunteers and you needed to jump on the ones you wanted to be a part of right away because they go fast. Other people may be interested in them as well.
Well i can tell you exactly what they do, because this is my son's second year at headstart. so this is my second time going to P O. Basically the teacher wants the parents to get familiar with their childs classroom, and most importantly the teacher. they will show you where his or her "cubby hole" is, the "naughty or nice" chart, and things like that. at some schools its required by law that you be shown the fire escape plan and things like that.
They will probably just let you know what to expect for the coming year. What your child will learn, how they handle kid's birthdays, pick a class mother, pick parents to help out at class parties, let you know about lunch and snack, etc. It's a chance for you to ask any questions too.