Maybe someone knows a tutorial where I can learn this effect. Example of this effect -I will be very grateful to anyone who can help
If you mean the fluctuating frequency of a note, that's called "vibrato".
If you mean fluctuating volume of a note, that's called "tremolo".
Vibrato is a pulsating sound effect produced by slight and rapid changes in the pitch (frequency) of a note. It has been used for centuries as a technique for adding expression and coloration to music, and it is characterized by two parameters, depth (the amount of pitch variation) and speed (how quickly the pitch is varied).
Tremolo, on the other hand, is a trembling or “shuddering” effect produced by slight and rapid changes in the volume (amplitude) of a note. It, too, has existed for hundreds of years as a musical technique, but for our purposes it is a much more recent technical innovation used in amplifier design. It is characterized by similarly labeled parameters, including depth (the amount of volume variation) and speed (how quickly fluctuations in volume are varied; also variously labeled as rate or intensity).
In short: Vibrato deals with change in pitch. Tremolo deals with change in volume.