Spare describes something that is extra. If you have a spare tire for your bike you can replace a flat, and if there's enough change in your pocket you can spare some to patch a friend's tire.
As a noun, a verb, and an adjective, the word spare has many meanings. A leftover part is a spare, you can spare a life when you save an animal or person, and you have spare time when you finish a project early. If you "spare no expense" you spend lots of money and when you spare a friend's feelings, you keep upsetting details to yourself. And if you have a moment to spare, you have time to share.