Pluck the expanded string (click it with your mouse)!
Observe the movement of the string particles!

After plucking only the two ends (C) of the string remain motionless and these are the oscillation nodes. The other parts of the string move simultaneously, performing a similar number of oscillations a second, so they have identical frequencies.

The maximum displacement of different parts of the string, the amplitude, is different during vibration. The part of the string having the biggest amplitude is called acoustic antinode.

Between the two nodes the string takes the form of a half-wave, so the length of the string is half the wavelength.