The Unit Circle is a circle with a radius of 1 and is centered at the coordinate point $(0,0)$. The unit circle is used to show the trigonometric functions of different angles. If we have a ray from the origin $(0,0)$ to a point on the circumference of the unit circle $(x,y)$ and it makes an angle $\theta$ from the positive x-axis, then we can say $\cos\theta=x$ and $\sin\theta=y.$
Below is a unit circle labeled with some of the more common angles you will encounter (in degrees and radians), the quadrant they are in(in roman numerals), and their associate sine and cosine values.
For example, if we take the angle $\theta = \frac{\pi}{6}$, we can tell that,
$$\cos{\frac{\pi}{6}} = \frac{\sqrt{3}}{2}$$
$$\sin{\frac{\pi}{6}} = \frac{1}{2}$$
The angle is in Quadrant I