Direct current, or DC, is a type of current in which electrons all flow in the same direction. It does not have to be a constant voltage - any absolute function can be a DC function. In other words, it never dips below the X axis of a voltage-time graph.
Alternating current, or AC, is a type of current in which the direction the electrons are flowing in rapidly changes.
The peak voltage is the maximum instantaneous voltage reached by an AC waveform. It can be thought of as the amplitude of the wave, assuming the wave is centred at the X axis without any deflection.
The RMS voltage of a sinusoidal AC waveform is the DC equivalent which gives the same amount of average power. It is always less than the peak voltage.
While it may be tempting to think of this as the average absolute voltage, or even just the average voltage, it is not the same!