It means you somehow influence them to get you a higher spot in admissions. The chances of getting a higher spot through traditional means (just visiting, touring the campus, etc) is pretty much negligible unless you're legacy but then that already carries some weight. In the end what they look at is your GPA, EC, etc. Just going for a campus tour will not really increase your chances of getting accepted. My friend toured Cambridge and he got to know the head dean pretty well but he still got rejected because of his GPA and lack of EC.
But who knows, you might pull something off with admissions/deans and they might really like you enough to give you priority admission. IF you do go, don't be pushy. Maybe try and get an interview or talk to someone in power. Just going for a tour will not do much, if at all.
tl;dr very slight increase but do not rely on that alone, some people get in the next day but most people do not.