The slash mark is shown multiple times in the piece, and it’s not just shown on the top line. The slash goes between notes and on any of the staff lines. I just want to know what it signifies.
It's a tiny caesura, often a breath mark. With bowing instructions penciled in, it wouldn't be a breath instruction even though the violinist apparently is considered to be able to catch enough of a breath to figure out whether they are bowing in the wrong direction: the immediately following note has an "upbow" instruction on it.