How can the unit circle be used to apply trigonometric functions to all real numbers?

The basic idea is that a complete turn around the unit circle has a length of 2 x pi (i.e., approximately 6.28). For numbers larger than 2 x pi, you go that distance around the unit circle, moving around it more than once - and eventually end up on some point on the unit circle. For example, if you go a distance of 3 x pi around the unit circle, that is equivalent of a distance of pi (equal to 180 degrees).

For negative numbers, you simply move around the unit circle in the opposite direction.