Each single ultrasonic pulse is composed of ultrasonic waves, the higher the frequency of these waves, the higher the frequency expressed in Hertz.
A lower frequency (e.g. 50 KHz) tends to penetrate deeper but discriminates less background detail. Conversely, a high frequency (e.g. 200 KHz) will find it difficult to reach high depths, but will be able to distinguish the bottom with much more detail and detail.
Until a few years ago, a lower frequency also meant a wider transmission angle, but this concept was somehow revolutionized by the evolution of the acoustics of a transducer, in fact today you can get wide transmission angles even at high frequencies, perhaps by inserting more elements in the transducer.
Today many fishfinders drive two frequencies (normally 50 KHz and 200 KHz), these can be selected as needed or appear simultaneously on the same screen, thus being able to distinguish from each individual peculiarity.