It's called slash notation and basically means "decide your own comping rhythm for the duration of each chord," commonly seen on jazz/popular music charts:
Slash notation is a form of purposefully vague musical notation which indicates or requires that an accompaniment player or players improvise their own rhythm pattern or comp according to the chord symbol given above the staff. On the staff a slash is placed on each beat (so that there are four slashes per measure in 4/4 time).
Slash notation and rhythmic notation may both be used in the same piece, for example, with the more specific rhythmic notation used in a section where the horn section is playing a specific melody or rhythmic figure that the pianist must support, and with slash notation written for the pianist for use underneath improvised solos.
It does not mean "play the chord on each slashed beat in a quarter note rhythm," it's just an indicator of how many beats long each chord is in effect (for whatever riffs/rhythms you want to do).
There are two kinds of slashes, or hash marks, used to indicate improvised chording or comping.
Citation based on Finale (Klemm)
(If you want to indicate ad lib comping, but you don’t require a specific rhythm, you can fill the measures with stemless slashes, spaced according to the time signature (four slashes in a measure, for example). If you want to indicate a specific rhythm of comping, you can turn the noteheads of any passage into slashes, still beamed and stemmed.)
Without more context, it looks like an F minor chord played 4 times each measure, medium loud.
From the revised take, the F minor measure is followed by a measure Dm7b5 (which could also be an Fm with a added D) followed by a measure of Db13. I'm not familiare with the convention for 13th chords (I assume this doesn't mean a tone cluster) so I don't know what tones are dropped. (To be fair, I find normal staff notation much easier to read when complicated chords arise.