Bold slashes are between every staff. I thought it may be pauses but it cannot be, wouldn't work. Maybe they are decoration, I don't know.
They are called "system dividers" or "system separators" and their purpose is to make it easier for the performers to find the next line of music, when the vertical spacing of the score is as tightly packed as your example.
They are more common in scores with more staves, or a variable number of staves, on each system. Four staves is about the minimum number where they make any difference to the readability.