Here's an example I just found online and there are many more in jazz lead sheets I found, etc. I was also wondering, when I'm writing a lead sheet and there are sections that are just instrumentals, do I put these lines, or not? What do the lines mean?
They're basically the rhythm pattern , the chords played essentially on guitar, but could be vamped on piano. They're sort of musical notes (crotchets), with no tails. Simply put, 4 in the bar. When I say basically, I mean a good start point for rhythm, signifying the simplest form the rhythm of that part can be. Of course, more accomplished players will extemporise on that, as and when the song calls for that.
No-one knows if the song's rhythm is straight or swung from that alone. Notice that in the end bar nothing is played on the first part of the first beat, and the first 'strum' is a shortened one, indicated by the staccato dot over the quaver. Having tails means that particular rhythm is the exact one the writer wants.
One site (Rebus) suggests that the slashes are to follow what's written in bar 1. As in a crotchet (proper), followed by 3 slashes, then 4 slashes per bar, would indicate 4 strums per bar (in 4/4 time) until changed by some other proper dot. Interesting? Never encountered that - yet.
It's called slash notation. Quote from the Dorico notation application's manual
Rhythm slashes are diagonal lines positioned on staves that are used to indicate that performers should play something, but without specifying the exact rhythms and pitches.
It means, "play something here". Each slash represents a time period of one quarter-note beat in your example picture.
In Finale the slashes are apparently also called "hash marks" https://usermanuals.finalemusic.com/Finale2014Win/Content/Finale/Slashes.htm
That page says that The number of hash marks is determined by the bottom number of the time signature. So if it was 6/8, you'd have 8 slashes?? I think it should be the top number of the time signature.
I tried to find what Elaine Gould's Behind Bars says about it, but couldn't find "slash" anything in the index.
These are rhythm indication for the chords above: it tells that the chords should be played two times a beat for the first seven bars for instance. Simply, four times per per bar for the first seven bars.
It is typical of chord chart notation.