For any polygon, the total degrees in the interior angles equals $180(n-2)$. A three-sided polygon has $180$ degrees. A four-sided one has $360$. Five sides gives $540$. Etc.
So why is it that $\lim_{n\to\inf}180(n-2)=360$? That is, why does a circle, an infinite-sided polygon, have the same number of degrees as a four-sided polygon?