Trip guarantees mean no surprises and no worries. :)
They create funding milestones by setting dates and the percentage of funding needed for each milestone. The milestones are set by the Trip Organizer and agreed to by trip members (you!) at the time of registration. A trip guarantee allows a trip organizer to have funds available when needed for items such as plane tickets, accommodation payments, and other hard costs that need to be paid in advance.
If fundraising goals aren't met in time, no worries! An individual member's credit card is charged for the milestone amount. This guarantees that you get to go on the trip even if the fundraising goals aren't met! Consider it an insurance policy for the mission.