I show you what Slash chords are and how we can use them in a musical context.
Using the G/F# as an example chord, we need to understand the first letter - G - means this is the chord we are playing everything other than the root note and the second letter - F# - means this is our root note now! To sum up, we have essentially changed the root note of a G chord.
In the context of a song these chords are mostly used as passing chords that land between to other chords, such as G going to G/F# and then Eminor.
Examples:
G/F# - changes G, G/F#, Em, Cadd9 - Song example: Songbird
G/B also common - Song examples: More Than Words and Country Roads
C/B - changes C, C/B, Am7, Fmaj7 - Song examples: Landslide and Wherever You Will Go
D/F# also common - Song examples: Thinking Out Loud and Highway To Hell
