The generator at the power station which produces our AC mains rotates through 360 degrees to produce one cycle of the sine wave form which makes up the supply.
In the next diagram there are two sine waves. They are out of phase because they do not start from zero at the same time. To be in phase they must start at the same time.
The waveform A starts before B and is LEADING by 90 degrees.
Waveform B is LAGGING A by 90 degrees.
The last diagram, known as a PHASOR DIAGRAM, shows this in another way. The phasors are rotating anticlockwise as indicated by the arrowed circle.
A is leading B by 90 degrees.
The length of the phasors is determined by the amplitude of the voltages A and B. Since the voltages are of the same value then their phasors are of the same length. If voltage A was half the voltage of B then its phasor would be half the length of B.
All this has nothing to do with "set your phasors on stun".