This is called "vibroacoustic signal". This kind of sound is called "vibrato". The kind in your music score is called "double vibrato" or "double vibrato" It means playing two notes first, then the last one, and alternating back and forth.

You should notice that there are four or two beats in that place, but there are two binary notes on the left hand in that paragraph, which seems to be a total of four beats. But in fact, music theory stipulates that the total duration of two notes marked with double vibrato is equal to the duration of one of them, that is, it is still two beats in total. Don't misunderstand it as four beats.

The number of oscillations (alternating times) of this vibrato is also particular, which is determined by the number of diagonal lines. Theoretically, if the diagonal number is 1, the eighth note will be vibrated; If the diagonal number is 2, vibrate with sixteenth notes; If the diagonal number is 3, it will vibrate 32 tones; ..... and so on. Because the vibrato mark has three diagonal lines, you should actually play it in thirty-two tones, which means playing it sixteen times in total, which means playing it eight times alternately. Specifically, if there are three diagonals between A and B (A is on the left and B is on the right), it should be played as: ABABABABABABAB (where A and B are thirty-two sounds).

Simply put, the number of diagonals is the same as the number of endings of the actually played notes (the thirty-second note has three endings), so it is represented by three diagonals.

Although the vibration must be uniform in theory, it can be used freely in practice.

If you don't understand anything, please ask me again. :)