In general, if $sin(\theta)= a$ then $\theta= sin^{-1}(a)$ and you can get that using a calculator. But a= 1/2 is a "special" situation. Imagine an equilateral triangle with side length 1. Each angle is $\pi/3$ radians or 180/3= 60 degrees. If you draw a line from one vertex to the midpoint of the opposite side, which is also perpendicular to the opposite side and bisects the vertex angle, you divide the equilateral triangle into two right triangles with hypotenuse of length 1 and one leg of length 1/2. That "leg of length 1/2" is half of the side that was bisected and is opposite the bisected angle. That angle is $\frac{\frac{\pi}{3}}{2}= \frac{\pi}{6}$ radians or $\frac{60}{2}= 30$ degrees. $\theta= sin^{-1}(1/2)= \frac{\pi}{6}$ radians.