Frequency within the scope of ultrasonic welding refers to the number of vibrations per second that the ultrasonic stack can produce.
Sound travels in a wave pattern, as the wave travels through space it moves a distance called amplitude. This distance is plus or minus a set amount. The frequency is rate at which the wave moves from one extreme to the other past a fixed nodal point.
In ultrasonic welding the frequency of the system can be abstracted to the number of vibration per second.