x = don't play string o = play open string
If the same fingering appears for more than one string, place the finger flat on the fingerboard as a 'bar', so all the strings can sound.
Examples of use
The G/F is effectively a G7 chord in 3rd inversion.
It is commonly used as a passing chord from I to IV.
In G major, that means it's a passing chord between G and C
