Single rooms are for people traveling on their own who prefer not to have a roomate.
The cost is more expensive because our trip costs with hotel accomodations are calculated based on double occupancy in every room. Single occupancy is a bit more expensive and thus the increased cost.
If you are traveling on your own, but do not mind having a roommate during your trip, you can select a standard "Land Only - Double Occupancy" cost and a roommate will be assigned. If there is no other singles on the trip that you can be paired up with, you might however be required to upgrade to a single occupancy room rate.