what does the notation with a slash mean in a chord

Slash chords just tell us what note to play in the bass. They’re usually pronounced as the chord name “over” the bass note. For example D7/F# we would say “D7 over F#” and it means play the F# as the lowest note.

D7 is quite a common one … play an open D7 chord like this

E |–2—-||
B |–1—-||
G |–2—-||
D |–0—-||
A |–X—-||
E |–X—-||

Now hook your thumb over and play the F# in the bass on the low E string.

E |–2—-||
B |–1—-||
G |–2—-||
D |–0—-||
A |–X—-||
E |–2—-||

That’s the basis of slash chords. Sometimes the note is not part of the chord, but often it is, which means we’re just playing an inversion.

Does that make sense?