The animation below shows the field amplitude as a function space along the axis of propagation. Each frame in the animation represents a time shift of 20 femtoseconds. The solution need not be spatially confined. For Fourier analysis of fields, unbounded harmonic solutions such as are popular. For such fields, one perceives wave propagation in the phase of the excitation.
Resonance refers to the tendency of systems to oscillate periodically at a single frequency.
Quantum mechanical resonances are important because these are the processes by which optical fields are generated. The electromagnetic field is the medium by which charges exert force on one another. Waves are generated in the field by moving charge. To generate a continuous wave, one moves the charge back and forth at a steady frequency. At radio frequencies, one can drive the oscillatory motion of charge using electrical circuits and one can radiate electromagnetic waves from the charge in antennas.