Input and Output Voltage of a Full bridge rectifier
Figure 1. Relationship between sinusoidal peak, RMS and average (after rectification) values. Image source: Learn About Electronics.
The peak voltage of a sinusoidal voltage waveform is \$ \sqrt 2 V_{RMS} \$. Conversely \$ V_{RMS} = \frac 1 {\sqrt 2} V_{peak} \$.
The RMS voltage can be considered that DC voltage which will give the same heating affect as the AC being considered. Since the power in a resistive load is given by \$ P = \frac {V^2} R \$ the RMS value is not the same as the average value.
Without smoothing capacitors the fullwave rectified DC will have the same RMS value as the AC waveform (ignoring losses in the rectifier).

Figure 2. Smoothing capacitor ripple. Image source: Electronics Notes.
The smoothing capacitor charges up on each peak of the rectified AC and supplies power to the circuit when the voltage falls away. See the linked article for how to size the smoothing capacitor.