It stands for Very Important Person. The guests you mark as VIP's are the people that just HAVE to be there. It's the birthday boy or girl at the birthday party; it’s the wedding couple at the engagement party; it's your three closest friends at the graduation party. Our software will weight VIP responses more heavily than other guests to help drive consensus around the dates that work for them.
Don't worry, the guests will not see who is and who isn't a VIP. Only the host can see which guests are marked VIPs. There is no difference in what the guest see.