$\displaystyle \pi/3$ is a common angle that we consider on the unit circle. Take a look at this:

File:Unit circle angles.svg - Wikipedia, the free encyclopedia
You probably have studied a version of this in a trig course.

The coordinate corresponding to $\displaystyle \pi/3$ is $\displaystyle (1/2,\sqrt{3}/2)$. That means $\displaystyle \cos(\pi/3)=1/2$ and $\displaystyle \sin(\pi/3)=\sqrt{3}/2$. We notice that $\displaystyle \sec x = \frac{1}{\cos x}$ and $\displaystyle \tan x = \frac{\sin x}{\cos x}$ and we can find each of those values in exact form. (You can also now see where the $\displaystyle \sqrt{3}$ term comes from.)