0 on non-open string note indicates that this note should be played using flageolet. In your example, there are three occurences of flageolet: the
d'' in the first bar and the
a'' and the last
d'' in the second. This is quite natural if you play this in the third position: with flageolet, you don't have to switch to the a string (or e string, in the second bar). Instead, the left hand remains in third position, only the Little finger (fourth finger) is placed on a different position than usual. This takes less time than switching both left hand and bow to the
In music notation, usually a small circle is used to notate a flageolet. But sometimes, instead of a circle, the
0 number is used.