Over one full sine wave cycle, the positive half and negative half are symmetric about the time axis. The positive area above the axis is equal in magnitude to the negative area below it, so they cancel each other.
Hence, the average value over a complete cycle becomes zero. The RMS value is obtained mathematically and equals the peak voltage divided by $\sqrt{2}$.
The rms value is $\frac{{\mathit{V}}_{\mathit{o}}}{\sqrt{2}}$ and the average value is 0.