The resonant frequency is the frequency at which a piezoelectric ceramic (crystal) will vibrate most efficiently. At the resonant frequency the crystal will produce the highest output with the least amount of voltage applied.

Usually, multiple probes are used because most transducers are only able to emit one frequency because the piezoelectric ceramic or crystals within it have a certain inherent frequency.
Multi-frequency probes have multiple crystals with different frequencies and the desired specific frequency can be selected. Advanced probes can emit sound waves at different frequencies for the near and far fields. The disadvantage is that multi-frequency (multifrequency) probes have slower frame rates and therefore they are only useful for imaging of static structures.

See also Dual Frequency Phased Array Transducer and Tri-Frequency Probe.