Voltage is always measured relative to a reference point, generally the reference point in a mains supply system is the "neutral" wire which we tie to the general mass of the earth.
Relative to the reference point the voltage at any instant can be either positive or negative, lets think of a DC system first, imagine we have one wire at +120V and another one at -120V. Now we have 240V between the +120V wire and the -120V wire.
Ok, what about AC? on AC the voltage is continuously changing in a (hopefully) sinusoidal pattern. There are various ways to characterise the voltage of an AC system, but the most common is RMS, the RMS voltage of an AC system is the equivalent DC voltage that would deliver the same average power into a resistive heating load.
Now some might say AC voltages don't have polarity and that is true when we look at a single hot wire in isolation, but when we look at more than one such supply there is something analogous to polarity, but rather than being 1D AC voltages (in a single frequency system) are 2D, they have a magnitude and a phase angle.
Just as we have to define a zero level for measuring any voltage, for measuring single-frequency AC voltages we have to define a zero phase angle. Usually we would define one of our phase conductors as being 0°, if the other supply is equal and opposite we consider it to be 180° out of phase.
This is the situation you have with the typical split single phase supplies used in north America. The transformer secondary is configured such that the two hots are equal and opposite.
Three phase wye supplies on the on the other hand have three hot conductors that are 120 degrees out of phase. Since the phases are only partially opposed the voltage between phases is only √3 times the phase to neutral voltage rather than double it.