When you spin, you whirl around in a circle. Many kids love to spin on a merry-go-round.
You can spin in a happy circle, and you can describe this movement as a spin. Another way to use the word spin is for a deliberate twisting of information, especially when it's done by politicians. A candidate might spin a news report to make herself look more appealing, for example, or spin bad news to make it appear good. This meaning of spin is relatively new, not being used much until the mid-1980's.