"Spare time" means "extra time, more than is needed."
"We're planning to get to Badlands National Park by Wednesday. If we have a little time to spare, we might stop and visit the Corn Palace on the way."
On a car, there may be four tires on the wheels and a fifth tire in the trunk; the fifth tire is the "spare tire."
When I go on a camping trip, there are batteries in my flashlight, but I also bring spare batteries.
"Do we have enough ice cream for Thanksgiving dinner?" "Yes, we have enough and to spare."